Heat releasing pipe

ABSTRACT

A heat releasing pipe includes a metal pipe and a surface coating layer. The metal pipe has an outer circumferential surface. The surface coating layer is provided on the outer circumferential surface of the metal pipe. The surface coating layer contains an inorganic glass base material and has concave portions and convex portions on an outer surface of the surface coating layer. The concave portions and the convex portions are constructed using electrocoating with an electrocoating resin. The concave portions have a virtually circular shape when seen in a direction perpendicular to the outer circumferential surface of the metal pipe and are lower than a first reference surface. The first reference surface has an average height of the outer surface of the surface coating layer. The convex portions are located on peripheral edge portions of the concave portions and surround the concave portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of the U.S. patentapplication Ser. No. 13/421,824 filed Mar. 15, 2012, which is issued asU.S. Pat. No. 9,188,251, and claims priority under 35 U.S.C. §119 toJapanese Patent Application No. 2011-057965, filed on Mar. 16, 2011. Thecontents of these applications are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a heat releasing pipe.

Discussion of the Background

In order to treat injurious substances such as injurious gases containedin exhaust gases discharged from an engine, a catalyst converter isinstalled in an exhaust gas passage including an exhaust pipe.

In order to improve the conversion efficiency of injurious substances bythe catalyst converter, it is necessary to maintain the temperature ofexhaust gases and the temperature of the exhaust pipe and the likethrough which the exhaust gases is allowed to pass at temperaturessuitable for activating the catalyst (hereinafter, referred to also as acatalyst activating temperature).

However, at the time of high speed operation of the engine, the exhaustgas temporarily has such a high temperature as to exceed 1000° C.Therefore, the temperature of the exhaust gases sometimes becomes higherthan the upper limit value of the catalyst activating temperature. As aresult, problematically, efficient purification of exhaust gases may bedifficult and the catalyst may deteriorate.

For this reason, an exhaust pipe connected to an automobile engine needsto be capable of externally radiating heat of the exhaust gases passingthrough the exhaust pipe at the time of high speed operation of theautomobile engine.

JP-A 2009-133213 and JP-A 2009-133214 have disclosed an exhaust pipehaving a structure in which a layer composed of a crystalline inorganicmaterial and an amorphous inorganic material is formed on a surface of acylindrical base material made of a metal.

JP-A 2009-133213 has disclosed an exhaust pipe having a structure inwhich a layer composed of a crystalline inorganic material and anamorphous inorganic material has infrared-ray emissivity higher thanthat of the base material so as to provide the exhaust pipe with anexcellent heat releasing characteristic.

Moreover, JP-A 2009-133214 has disclosed an exhaust pipe having astructure in which the amorphous inorganic material located on the outerperipheral side from the crystalline inorganic material has an averagethickness of 20 μm or less so as to provide the exhaust pipe with anexcellent heat releasing characteristic.

The contents of JP-A 2009-133213 and JP-A 2009-133214 are incorporatedherein by reference in their entirety.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a heat releasing pipeincludes a metal pipe and a surface coating layer. The metal pipe has anouter circumferential surface. The surface coating layer is provided onthe outer circumferential surface of the metal pipe. The surface coatinglayer contains an inorganic glass base material and has concave portionsand convex portions on an outer surface of the surface coating layer.The concave portions and the convex portions are constructed usingelectrocoating with an electrocoating resin. The concave portions have avirtually circular shape when seen in a direction perpendicular to theouter circumferential surface of the metal pipe and are lower than afirst reference surface. The first reference surface has an averageheight of the outer surface of the surface coating layer. The convexportions are located on peripheral edge portions of the concave portionsand surround the concave portions. The convex portions are higher thanthe first reference surface.

BRIEF DESCRIPTIONS OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

FIG. 1A is an explanatory drawing that schematically illustrates aconcave portion in an exhaust pipe of the embodiment of the presentinvention. FIG. 1B is an explanatory drawing that schematicallyillustrates a peripheral edge portion in the exhaust pipe of theembodiment of the present invention. FIG. 1C is an explanatory drawingthat schematically illustrates a concave portion in the exhaust pipe ofthe embodiment of the present invention.

FIG. 2 is a perspective view that schematically illustrates an exhaustpipe of the embodiment of the present invention.

FIG. 3 is a partially enlarged cross-sectional view that schematicallyillustrates a cross section obtained by cutting the exhaust pipe shownin FIG. 2 in its longitudinal direction.

FIG. 4A is a partially enlarged cross-sectional view that illustrates across section obtained by cutting the exhaust pipe shown in FIG. 2 inits longitudinal direction. FIG. 4B is a partially enlargedcross-sectional view that illustrates a cross section obtained bycutting the exhaust pipe shown in FIG. 2 in its longitudinal direction.

FIG. 5 is a photograph of the exhaust pipe of FIG. 2, taken in thedirection perpendicular to the surface of the metal base material.

FIG. 6A is an explanatory drawing that schematically illustrates ananionic electrocoating process according to an embodiment of the presentinvention. FIG. 6B is an explanatory drawing that schematicallyillustrates a cationic electrocoating process according to theembodiment of the present invention.

FIG. 7A to FIG. 7E are explanatory drawings that illustrate an exemplarymanufacturing procedure of the exhaust pipe of the embodiment of thepresent invention.

FIG. 8 is an explanatory drawing that illustrates a correspondingrelationship between a pH value and an electric charge with respect toinorganic glass particles and inorganic particles according to anembodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

The inventions disclosed in JP-A 2009-133213 and JP-A 2009-133214 eachhave realized an exhaust pipe having an excellent heat releasingcharacteristic. However, there have been still strong demands for anexhaust pipe that is still superior in heat releasing characteristic,and in particular, the development of an exhaust pipe that is by farsuperior in heat releasing characteristic.

Namely, an exhaust pipe according to an embodiment of the presentinvention includes: a metal base material; and a surface coating layerformed on a surface of the metal base material, wherein the surfacecoating layer contains an inorganic glass base material, the surfacecoating layer has a concave portion on a surface of the surface coatinglayer, the concave portion being lower than a first reference surfacehaving an average height of the surface of the surface coating layer,and the concave portion is surrounded by a convex portion present on aperipheral edge portion of the concave portion, the convex portion beinghigher than the first reference surface.

Since the exhaust pipe according to the embodiment of the presentinvention has concave portions and convex portions formed on the surfaceof its surface coating layer, the surface area of the exhaust pipebecomes great so that the exhaust pipe is likely to have high apparentemissivity. For this reason, by accelerating the radiant heat transfer,the resultant exhaust pipe is likely to have an excellent heat releasingcharacteristic.

Moreover, the concave portions formed on the surface of the surfacecoating layer are likely to serve as a number of non-fixed ends fordispersing thermal stress. Furthermore, by the concave portions formedon the surface of the surface coating layer, portions having smallerfilm thicknesses are formed in the surface coating layer. Since atemperature difference in each of these portions becomes small in thethickness direction, thermal stress hardly occurs inside the surfacecoating layer. Therefore, the thermal stress due to thermal impact iseasily relieved to easily prevent separation of the surface coatinglayer. As a result, the exhaust pipe easily maintains a high heatreleasing characteristic.

The following description will discuss the “peripheral edge portion”.

FIG. 1A is an explanatory drawing that schematically illustrates aconcave portion in the exhaust pipe according to the embodiment of thepresent invention.

FIG. 1B is an explanatory drawing that schematically illustrates aperipheral edge portion in the exhaust pipe according to the embodimentof the present invention.

FIG. 1A schematically illustrates an appearance of a concave portion 30when a surface coating layer is observed in the direction perpendicularto the surface of a metal base material. The concave portion 30 forms arounded graphic form 31.

FIG. 1B illustrates a graphic form 61 that is similar to the graphicform 31. The center of gravity of the graphic form 61 is coincident withthe center of gravity of the graphic form 31. The similar ratio (thegraphic form 31:the graphic form 61) is r:r′, and r′=1.2r is satisfied.

A peripheral edge portion 60 is an area corresponding to an area insidethe graphic form 61 from which the concave portion 30 is excluded.

In the exhaust pipe according to the embodiment of the presentinvention, the convex portions are present on the peripheral edgeportion of each concave portion in a manner of surrounding the concaveportion. Here, the phrase “the convex portions are present in a mannerof surrounding the concave portion” refers to a state where about 60% ormore of the peripheral edge portion of the concave portion is occupiedby the convex portion. Among the areas of the peripheral edge portion ofthe concave portions, a ratio of the area occupied by the convexportions is desirably about 80% or more, more desirably about 90% ormore, and most desirably 100%. This is because the surface area of theexhaust pipe becomes larger along with the increase in the ratio of thearea occupied by the convex portions among the areas of the peripheraledge portion. In the case where the ratio of the area occupied by theconvex portions among the areas of the peripheral edge portion is about60% or more, the heat releasing characteristic of the exhaust pipe islikely to be improved because the surface area of the exhaust pipe islarge.

Additionally, the concave portion according to the embodiment of thepresent invention is an area having a peripheral edge portion in whichthe convex portions are present, among areas lower than the firstreference surface. That is, even in the case where there is an arealower than the first reference surface, if the area has no convexportion on its peripheral edge portion, the area is not a concaveportion.

Therefore, in the embodiment of the present invention, all areas lowerthan the first reference surface are not necessarily required to haveconvex portions in their peripheral portions. Here, supposing that theareas lower than the first reference surface are referred to aspotential concave portions, it is only necessary that the potentialconcave portions includes an area having convex portions on itsperipheral edge portion in the embodiment of the present invention.

In the exhaust pipe according to the embodiment of the presentinvention, the convex portion is preferably higher than a secondreference surface having a height of (H_(max)−H×⅓), wherein H_(max)represents a maximum value of the height of the surface coating layer,H_(min) represents a minimum value of the height of the surface coatinglayer, and H represents a difference between H_(max) and H_(min).

In the exhaust pipe, convex portions higher than the second referencesurface are present on the peripheral edge portion. The second referencesurface is a face having a height of (H_(max)−H×⅓). Namely, in theexhaust pipe, an area much higher than the concave portion is present onthe peripheral edge portion of the concave portion (i.e. comparativelyin the vicinity of the concave portion). Therefore, the surface coatinglayer forms a steep slope from the concave portion to the convexportion. This increases the surface area to increase the apparentemissivity. This increase easily improves the heat releasingcharacteristic.

In the exhaust pipe according to the embodiment of the presentinvention, the concave portion preferably has a virtually circular shapewhen seen in a direction perpendicular to the surface of the metal basematerial.

This presumably allows the thermal stress due to thermal impact to beeasily relieved. In the case where separation has once occurred at anedge portion of the concave portion, if the concave portion has a linearshape, the separation proceeds successively; however, in the case wherethe concave portion has a virtually circular shape, the separation isless likely to proceed because the separated surface coating layer ispulled by the surface coating layer therearound. As a result, theadhesion between the surface coating layer and the metal base materialis likely to be improved.

The following description will discuss “virtually circular shape”.

FIG. 1C is an explanatory view that schematically illustrates a concaveportion in the exhaust pipe according to the embodiment of the presentinvention.

FIG. 1C shows the same concave portion 30 as the concave portion 30shown in FIG. 1A.

In FIG. 1C, r_(max) represents the maximum value of a distance between apoint on the graphic form 31 and the center of gravity 50, and r_(min)represents the minimum value of a distance between a point on thegraphic form 31 and the center of gravity 50.

The “virtually circular shape” refers to the case where a relationshipof r_(max)<about 1.5 r_(min) is satisfied.

In the exhaust pipe according to the embodiment of the presentinvention, d>0 is preferably satisfied, wherein H_(min) represents aminimum value of the height of the surface coating layer and drepresents a distance between a face having a height of H_(min) and thesurface of the metal base material.

In the case of d>0, namely, in the case where the metal base material isnot exposed on the surface of the exhaust pipe, the effect of improvingthe emissivity by the concave portion formed on the surface of thesurface coating layer is likely to be achieved sufficiently. Moreover,since the metal base material has low emissivity, the effect ofimproving the emissivity is presumably less likely to be reduced. In thecase of d>0, the metal base material having low emissivity is notexposed on the surface, and therefore, deterioration in the heatreleasing characteristic is likely to be avoided.

In the exhaust pipe according to the embodiment of the presentinvention, d≧about 2 μm is preferably satisfied.

A predetermined distance (about 2 μm) or more between the surface of themetal base material having low emissivity and the bottom of the concaveportion is likely to provide a sufficient effect of improving theemissivity by the concave portion formed on the surface of the surfacecoating layer, resulting in high emissivity. Accordingly, deteriorationin the heat releasing characteristic is likely to be more effectivelyavoided.

In the exhaust pipe according to the embodiment of the presentinvention, the concave portion preferably has a virtually circular shapehaving a diameter of about 3 μm to about 2000 μm when seen in adirection perpendicular to the surface of the metal base material.

As mentioned above, given that an increase in surface area of thesurface coating layer contributes to an improvement in emissivity,desirably, the size of the concave portion is small and the densitythereof is high.

However, in the case where the size of the concave portion is too small,the walls of the concave portion are made face to face with each otherclosely. In such a case, infrared rays radiated upon heating of thesurface coating layer are hardly radiated outside of the surface coatinglayer, resulting in low heat releasing effect. On the other hand, sincethe emissivity at the concave portion is low corresponding to the smallthickness of the surface coating layer, the emissivity of the entiresurface coating layer is lowered when the size of the concave portion istoo large, leading to a case where a high heat releasing characteristicis less likely to be obtained.

In the exhaust pipe according to the embodiment of the presentinvention, since the concave portion has an appropriate size (about 3 μmto about 2,000 μm in diameter of circle), the exhaust pipe is likely tohave an excellent heat releasing characteristic.

In the exhaust pipe according to the embodiment of the presentinvention, the density of the concave portions is preferably about 10pcs/cm² to about 10⁷ pcs/cm².

Given that an increase in surface area of the surface coating layercontributes to an improvement in emissivity, as above mentioned, thedensity of the concave portions is desirably high. In the case where thedensity of the concave portions is too low, since an increase in surfacearea is small, the effect for improving the emissivity is hardlyobtained.

On the other hand, in the case where the density of the concave portionsis too high, two concave portions are positioned too close to each otherso that they may be partially overlapped with each other. When the twoconcave portions are overlapped with each other, a convex part is formedbetween the two concave portions. Since this convex part is lower thanthe first reference surface, such a convex part is not theaforementioned convex portion, and is not continuously formed in amanner of surrounding the concave portion. Consequently, the convex parttends to be a portion that is easily separated. For this reason,separation occurs from the convex part as a starting point with anelapse of time, and the emissivity may possibly be lowered.

In the exhaust pipe according to the embodiment of the presentinvention, since concave portions are formed at an appropriate density,the exhaust pipe is likely to have an excellent heat releasingcharacteristic.

In the exhaust pipe according to the embodiment of the presentinvention, the surface coating layer preferably further containsinorganic particles.

Since the inorganic particles are highly emissive, infrared rays arereleased strongly upon heating. This is indicated by Stefan-Boltzmannlaw represented by the following equation (1):q=εσ(T ₁ ⁴ −T ₂ ⁴)  (4)(σ: Stefan-Boltzmann constant . . . 5.67×10⁻⁸ [w/m²·K⁴], q: heat flux[W/m²], ε: emissivity, T₁: heating unit temperature [K], T₂: heatreceiving unit temperature [K]).

Therefore, in an exhaust pipe containing inorganic particles in thesurface coating layer, infrared rays are emitted from the inorganicparticles in the surface coating layer. Then, the emissivity of thesurface coating layer becomes high so that such an exhaust pipe islikely to have an excellent heat releasing characteristic at hightemperature.

In the exhaust pipe according to the embodiment of the presentinvention, the inorganic particles preferably have an average particlesize of not more than about 3 μm.

As mentioned above, the inorganic particles have a function forimproving emissivity. For this reason, in the case where portions wherethe inorganic particles are present are projected onto a plane inparallel with the surface of the metal base material, the emissivitybecomes greater along with the increase in the area of the projectedportions.

If the average particle size of the inorganic particles is great, theinorganic particles are localized in some areas, while the other areaslack the inorganic particles. In this case, the above area is small.Consequently, the emissivity is lowered.

That is, in the case where the ratio of the inorganic particlescontained in the surface coating layer is constant, the area becomeslarger along with the reduction in the average particle size of theinorganic particles.

In the exhaust pipe according to the embodiment of the presentinvention, since the inorganic particles having an average particle sizeof about 3 μm or less are used, the exhaust pipe is likely to have anexcellent heat releasing characteristic at high temperature.

In the exhaust pipe according to the embodiment of the presentinvention, the inorganic particles preferably have an averageinterparticle distance of not more than about 3 μm.

As mentioned above, the inorganic particles have a function forimproving emissivity. For this reason, in the case where portions wherethe inorganic particles are present are projected onto a plane inparallel with the surface of the metal base material, the emissivitybecomes greater along with the increase in the area of the projectedportions.

If the interparticle distance of the inorganic particles is great, theinorganic particles are localized in some areas, while the other areaslack the inorganic particles. In this case, the above area is small.Consequently, the emissivity is lowered.

That is, in the case where the ratio of the inorganic particlescontained in the surface coating layer is constant, the area becomeslarger along with the reduction in the interparticle distance of theinorganic particles.

In the exhaust pipe according to the embodiment of the presentinvention, since the average interparticle distance of the inorganicparticles is small as 3 μm, the exhaust pipe is likely to have anexcellent heat releasing characteristic at high temperature.

In the exhaust pipe according to the embodiment of the presentinvention, the inorganic particles are preferably oxides of a transitionmetal.

In the exhaust pipe according to the embodiment of the presentinvention, the inorganic glass base material preferably has a softeningpoint of about 300° C. to about 1000° C.

A description is given on an exhaust pipe according to one embodiment ofthe present invention.

FIG. 2 is a perspective view that schematically illustrates an exhaustpipe according to the embodiment of the present invention.

FIG. 3 is a partially enlarged cross-sectional view that schematicallyillustrates a cross section obtained by cutting the exhaust pipe shownin FIG. 2 in its longitudinal direction.

In FIG. 2, an exhaust gas is indicated by G and the flowing direction ofthe exhaust gas is indicated by arrows.

An exhaust pipe 1 shown in FIG. 2 is constituted by a substantiallycylindrical metal base material 10 and a surface coating layer 20 formedon the outer peripheral face of the metal base material 10 with apredetermined thickness.

Examples of the material for the metal base material include, but arenot particularly limited to; metals such as stainless steel, steel, ironand copper; and nickel alloys such as Inconel, Hastelloy and Invar.Since these metal materials have high thermal conductivity, use of anyof these is likely to contribute to an improvement in heat releasingcharacteristic of an exhaust pipe.

As shown in FIG. 3, on the surface of the metal base material 10,irregularities are desirably formed. The surface roughness Rz_(JIS) (JISB 0601: 2001) of the outer peripheral face of the metal base materialhaving these irregularities is desirably about 1.5 μm to about 15.0 μm.

In the case where the surface roughness Rz_(JIS) of the outer peripheralface of the metal base material is about 1.5 μm or more, since thesurface area of the metal base material is large, the adhesion betweenthe metal base material and the surface coating layer is likely to beachieved. On the other hand, in the case where the surface roughnessRz_(JIS) of the outer peripheral face of the metal base material isabout 15.0 μm or less, voids are less likely to be formed between thesurface of the metal base material and the surface coating layer. Thisis presumably because when the surface roughness Rz_(JIS) of the outerperipheral face of the metal base material is not too high, the painteasily enters the concave portions of the irregularities formed on thesurface of the metal base material. If the voids are formed between thesurface of the metal base material and the surface coating layer, theadhesion between the metal base material and the surface coating layerbecomes insufficient.

The surface coating layer 20 contains an inorganic glass base material.

The inorganic glass base material is preferably a low melting glasshaving a softening point of about 300° C. to about 1000° C. Examples ofthe low melting glass include, but are not particularly limited to,soda-lime glass, non-alkali glass, borosilicate glass, potash glass,crystal glass, titanium crystal glass, barium glass, boron glass,strontium glass, alumina silicate glass, soda zinc glass, and sodabarium glass. These glasses may be used alone, or two or more kinds ofthese may be used in combination.

In the case where the above-mentioned low melting glass has a softeningpoint in a range from about 300° C. to about 1000° C., a surface coatinglayer is likely to be firmly and easily formed on the outer peripheralface of the base material by application (coating) of the molten lowmelting glass onto the outer peripheral face of a metal base materialfollowed by a heating and firing treatment thereof.

In the case where the softening point of the low melting glass is about300° C. or higher, the low melting glass used in an exhaust pipe doesnot easily soften upon application of heat. In such a case, whenexternal foreign matters such as stone and sand are brought into contactwith the softened glass, they tend not to be easily attached to theglass. If the foreign matters are attached to the surface, the surfacecoating layer having high radiation rate is covered with the foreignmatters, failing to provide an exhaust pipe having an excellent heatreleasing characteristic at high temperature.

On the other hand, in the case where the softening point of the lowmelting glass is about 1000° C. or lower, since a heating treatment maybe conducted at a heating temperature of less than about 1000° C., themetal base material is less likely to deteriorate due to exposure tohigh temperature in the heating treatment upon forming a surface coatinglayer of the exhaust pipe.

Additionally, the softening point of the low melting glass can bemeasured by using, for example, an automatic measuring apparatus ofglass softening and strain points (SSPM-31) manufactured by OPTCorporation, in accordance with a method according to JIS R 3103-1:2001.

Examples of the borosilicate glass include, but are not particularlylimited to, SiO₂—B₂O₃—ZnO glass and SiO₂B₂O₃—Bi₂O₃ glass. The crystalglass refers to glass containing PbO, and examples thereof include, butare not particularly limited to, SiO₂—PbO glass, SiO₂—PbO—B₂O₃ glass,and SiO₂—B₂O₃—PbO glass. Examples of the boron glass include, but arenot particularly limited to, B₂O₃—ZnO—PbO glass, B₂O₃—ZnO—Bi₂O₃ glass,B₂O₃—Bi₂O₃ glass, and B₂O₃—ZnO glass. Examples of the barium glassinclude, but are not particularly limited to, BaO—SiO₂ glass.

The surface coating layer 20 desirably contains inorganic particles.

As the inorganic particles, particles of an oxide of a transition metalare desirably used. More desirably, the oxide is at least one kind ofoxides of manganese, iron, copper, cobalt, chromium and nickel.

These inorganic particles may be used alone, or two or more of these maybe used in combination.

Since the oxides of these transition metals are highly emissive,infrared rays are strongly irradiated upon heating so that the heatreleasing characteristic of the exhaust pipe owing to radiant heattransfer is likely to be improved.

The inorganic particles in the surface coating layer 20 desirably havean average interparticle distance of about 3 μm or less.

The inorganic particles have a function for improving emissivity. Forthis reason, in the case where portions where the inorganic particlesare present are projected onto a plane in parallel with the surface ofthe metal base material, the emissivity becomes greater along with theincrease in the area of the projected portions.

If the interparticle distance of the inorganic particles is great, theinorganic particles are localized in some areas, while the other areaslack the inorganic particles. In this case, the above area is small.Consequently, the emissivity is lowered.

That is, in the case where the ratio of the inorganic particlescontained in the surface coating layer is uniform, the area becomeslarger along with the reduction in the interparticle distance of theinorganic particles.

In the case where the inorganic particles have an average interparticledistance of about 3 μm or less, since the interparticle distance of theinorganic particles is not too large, an exhaust pipe 1 is likely tohave a desired heat releasing characteristic.

However, the inorganic particles in the surface coating layer may havean average interparticle distance of exceeding about 3 μm.

The inorganic particles in the surface coating layer have an averageinterparticle distance of desirably about 0.1 μm or more. If the averageof the interparticle distance is about 0.1 μm or more, thermal stress isnot great in areas among the particles upon heating and cooling, whichmay not cause a crack in an inorganic glass material. When a crackoccurs in the inorganic glass material, another crack may be caused inthe surface coating layer starting from the crack and dropping off ofthe layer may be caused, failing to provide an exhaust pipe having highemissivity.

As shown in FIG. 3, concave portions 30 and convex portions 40 arepresent on the surface of the surface coating layer 20.

Referring to FIG. 4A, FIG. 4B, and FIG. 5, the following descriptionwill discuss the concave portions 30 and the convex portions 40.

FIG. 4A and FIG. 4B are partially enlarged cross-sectional views, eachillustrating a cross section obtained by cutting the exhaust pipe shownin FIG. 2 in its longitudinal direction.

FIG. 5 is a photograph of the exhaust pipe of FIG. 2, taken in thedirection perpendicular to the surface of the metal base material.

The photograph shown in FIG. 5 is taken by an electron microscope at anaccelerating voltage of 15.0 kV and at 200× magnification.

As shown in FIG. 4A and FIG. 4B, the concave portions 30 correspond toareas lower than a first reference surface, and the first referencesurface is a surface having an average height of the surface of thesurface coating layer 20.

In FIG. 4A and FIG. 4B, the first reference surface represents anaverage line.

The average line corresponds to an average line for use in defining across-sectional curve in accordance with JIS B601 (2001), and is a curverepresenting a nominal profile applied to the cross-sectional curveusing a least square method.

The nominal profile indicates a shape such as an inclination of a plane,and an arc shape of a cylindrical part.

The average line is automatically calculated by measuring a surfaceshape using a commercially available surface roughness measuring device(such as Wyko NT9100 (optical device), manufactured by VeecoInstruments) to obtain a cross-sectional curve.

The concave portion desirably has a virtually circular shape when seenin the direction perpendicular to the surface of the metal basematerial.

In the case where separation has once occurred at an edge portion of theconcave portion, if the concave portion has a linear shape, theseparation successively proceeds; however, in the case where the concaveportion has a virtually circular shape, the separation hardly proceedsbecause the separated surface coating layer is pulled by the peripheralsurface coating layer. Therefore, when the concave portion has avirtually circular shape, the adhesion between the surface coating layerand the metal base material is likely to be improved.

As shown in FIG. 5, the concave portion 30 has a round shape (virtuallycircular shape) when seen in the direction perpendicular to the surfaceof the metal base material 10. In the present description, such a shapeof the concave portion 30 is referred to as a “virtually circularshape”.

When the shape of the concave portion viewed in the directionperpendicular to the surface of the metal base material is substantiallya circle (virtually circular shape), the diameter of the circle isdesirably about 3 μm to about 2000 μm.

Given that an increase in surface area of the surface coating layercontributes to an improvement in emissivity, the size of the concaveportion is desirably small and the density thereof is desirably high.

However, in the case where the size of the concave portion is too small,the walls of the concave portion are made face to face with each otherclosely. In such a case, infrared rays radiated upon heating of thesurface coating layer are hardly radiated outside of the surface coatinglayer so that the heat releasing effect becomes small. On the otherhand, since the emissivity at the concave portion is low correspondingto the small thickness of the surface coating layer, the emissivity ofthe entire surface coating layer is lowered when the size of the concaveportion is too large, leading to a case where a high heat releasingcharacteristic is less likely to be obtained.

In the case where the diameter of the circle is from about 3 μm to about2000 μm, an obtained exhaust pipe is likely to be excellent in heatreleasing characteristic.

The diameter of the circle is more desirably about 1000 μm or less, andfurthermore desirably about 120 μm or less.

The diameter of the circle corresponds to the maximum length of astraight line drawn inside the circle.

The density of the concave portions is desirably about 10 pcs/cm² toabout 10⁷ pcs/cm².

Given that an increase in surface area of the surface coating layercontributes to an improvement in emissivity, the density of the concaveportions is desirably high. In the case where the density of the concaveportions is too low, since an increase in surface area is small, theeffect for improving the emissivity is hardly obtained.

On the other hand, in the case where the density of the concave portionsis too high, two concave portions are made too close to each other sothat they may be partially overlapped with each other. When the twoconcave portions are overlapped with each other, a convex part is formedbetween the two concave portions. Since this convex part is lower thanthe first reference surface, this convex part is not the aforementionedconvex portion, and is not continuously formed in a manner ofsurrounding the concave portion. Consequently, the convex part tends tobe a portion that is easily separated. For this reason, separationoccurs from the convex part as a starting point with an elapse of time,and the emissivity may possibly be lowered.

In the case where the density of the concave portions is from about 10pcs/cm² to about 10⁷ pcs/cm², an obtained exhaust pipe is likely to havean excellent heat releasing characteristic.

The density of the convex portions is more desirably about 1×10²pieces/cm² or more, and furthermore desirably about 5×10² pcs/cm² ormore.

Supposing that the distance between the surface having a height ofH_(min) (see FIG. 4A and FIG. 4B) and the surface of the metal basematerial is defined as d (see FIG. 3), d>0 is desirably satisfied, andmore desirably, d≧about 2 μm is satisfied. H_(min) is the minimum valueof the surface height of the surface coating layer.

In the case of d>0, since the metal base material is not exposed on thesurface of the exhaust pipe, the effect for improving the emissivityexerted by the concave portions formed on the surface of the surfacecoating layer is likely to be sufficiently obtained. Moreover, since theemissivity of the metal base material exposed on the surface is low, theeffect for improving the emissivity is presumably less likely to besmall. Therefore, in the case of d>0, lowering of the releasingcharacteristic is easily prevented.

In the present description, the distance d between the surface havingthe height of H_(min) and the surface of the metal base material isreferred to also as “film thickness of the concave portion”. Moreover, adistance D between the surface having the height of H_(max) and thesurface of the metal base material is referred to also as “filmthickness of the surface coating layer”.

Moreover, in the case of film thickness of the concave portion d=0, sucha state is referred to as “the concave portion penetrates the surfacecoating layer”.

As shown in FIG. 4A and FIG. 4B, the convex portions 40 are areas higherthan a second reference surface. The second reference surface is asurface having a height of (H_(max)−H×⅓).

H_(max) is the maximum value of the surface height of the surfacecoating layer 20. H is a difference between H_(max) and H_(min) andH_(min) is the minimum value of the surface height of the surfacecoating layer 20.

As shown in FIG. 4A and FIG. 4B, the surface coating layer forms a steepslope from a concave portion to a convex portion.

In this case, H_(max) is a height of the highest point in the entiresurface of the surface coating layer 20. H_(min) is a height of thelowest point in the entire surface of the surface coating layer 20.

FIG. 4A shows the case where the highest point 45 in the cross sectioncorresponds to the highest point in the entire surface of the surfacecoating layer 20, and the lowest point 35 in the cross sectioncorresponds to the lowest point in the entire surface of the surfacecoating layer 20.

FIG. 4B shows the case where the highest point in the entire surface ofthe surface coating layer 20 is higher than the highest point 45 in thecross section, and the lowest point in the entire surface of the surfacecoating layer 20 is lower than the lowest point 35 in the cross section.

The above description has discussed the concave portions 30 and theconvex portion 40 of the exhaust pipe 1 according to the embodiment ofthe present invention.

In the present embodiment, convex portions are present on the peripheraledge portion of each concave portion. A description has been alreadygiven on the peripheral edge portion with reference to FIG. 1A and FIG.1B.

A description is given on a method for manufacturing the exhaust pipeaccording to the embodiment of the present invention.

In a method for manufacturing the exhaust pipe according to theembodiment of the present invention, a predetermined paint is used.

The paint in accordance with the present embodiment will be described inthe following.

The paint in accordance with the exhaust pipe according to theembodiment of the present invention contains inorganic glass particlesand an electrocoating resin.

In the method for manufacturing the exhaust pipe according to theembodiment of the present invention, a coat film is formed on a surfaceof a metal base material by carrying out electrocoating using a paintcontaining an electrocoating resin. Thereafter, the coat film is heatedto a temperature that is not lower than the burning-out temperature ofthe electrocoating resin, and the coat film is then further heated to atemperature that is not lower than the softening point of inorganicglass particles. As a result, a surface coating layer having concaveportions formed on its surface is formed.

Referring to FIG. 6A, FIG. 6B, and FIG. 7A to FIG. 7E, the followingdescription will discuss the reason why the concave portions are formedon the surface of the exhaust pipe manufactured using the paintaccording to the exhaust pipe according to the embodiment of the presentinvention.

FIG. 6A is an explanatory drawing that schematically illustrates theprocess of anionic electrocoating according to the embodiment of thepresent invention.

FIG. 6B is an explanatory drawing that schematically illustrates theprocess of cationic electrocoating according to the embodiment of thepresent invention.

FIG. 7A to FIG. 7E are explanatory drawings that illustrate an exemplarymanufacturing procedure of the exhaust pipe according to the embodimentof the present invention.

As shown in FIG. 6A and FIG. 6B, the electrocoating process includes ananionic electrocoating process and a cationic electrocoating process.

Additionally, in the example shown in FIG. 7A to FIG. 7E, an anionicelectrocoating resin is used as the electrocoating resin; however, acationic electrocoating resin may also be used as the electrocoatingresin. Moreover, a paint and a surface coating layer contain inorganicparticles in the example; however, inorganic particles are notnecessarily required to be contained.

In the anionic electrocoating process, an anionic electrocoating resinis used as the electrocoating resin.

The anionic electrocoating resin has a functional group (for example,carboxyl group) that reacts with a base to form a salt and is negativelycharged by neutralization with a base (for example, organic amine) (seethe following formula (2)):R—COOH+NR₃→R—COO⁻+NR₃H⁺  (2)

A metal base material and an electrode plate are placed in anelectrocoating vessel and a current is applied thereto so that theelectrocoating resin negatively charged is attracted toward the anode(see FIG. 6A). Then, inorganic glass particles and the like contained ina paint are carried onto the surface of the metal base material (objectto be coated) together with the electrocoating resin. When theelectrocoating resin is brought into contact with the surface of themetal base material, the following reactions (3) and (4) proceed:2H₂O→4H⁺+4e ⁻+O₂←  (3)R—COO⁻+H⁺→R—COOH  (4)Consequently, since the electrocoating resin becomes insoluble, theinorganic glass particles and the like are deposited on the surface ofthe metal base material (anode).

In contrast, in the cationic electrocoating process, a cationicelectrocoating resin is used as the electrocoating resin.

Since the cationic electrocoating resin is positively charged to beattracted toward the cathode (see FIG. 6B), the inorganic glassparticles and the like are deposited on the surface of the metal basematerial (cathode).

As described above, in the electrocoating process, the electrocoatingresin carries the inorganic glass particles and the like onto thesurface of the metal base material (see FIG. 7A). Then, theelectrocoating resin is brought into contact with the surface of themetal base material, and deposited on the surface of the metal basematerial (see FIG. 7B and FIG. 7C). At this time, paths through whichthe soluble electrocoating resin is allowed to pass are formed in thecoat film (see FIG. 7B and FIG. 7C). As shown in the reaction formula(3), when the electrocoating resin is brought into contact with thesurface of the metal base material, an oxygen gas is generated. Thesepaths are formed when the oxygen gas thrusts the coat film that hasalready been formed aside to get into the coating solution. In the casewhere the electrocoating resin remains inside the paths after theelectrocoating process, since the electrocoating resin is not depositedon the surface of the metal base material and is soluble, the resin maybe washed away with water.

Thereafter, when the coat film is heated, the electrocoating resin isburned out so that the volume of the coat film is contracted. Duringthis process, the concave portions are formed in accordance with thepaths (see FIG. 7D and FIG. 7E).

The description has been given on how the concave portions are formed onthe surface of the exhaust pipe according to the embodiment of thepresent invention, which is manufactured using the paint according tothe exhaust pipe according to the embodiment of the present invention.

The description is continuously given on the paint according to theexhaust pipe according to the embodiment of the present invention.

The average particle size of the inorganic glass particles is notparticularly limited, and is preferably not larger than about 3 μm.

In the case where the average particle size of the inorganic glassparticles is about 3 μm or less, the sizes of particles are less likelyto be varied. In this case, during the electrocoating process, since theflow of the coating film is less likely to be locally disturbed, pathsare easily formed. As a result, concave portions are likely to be formedon the surface of the exhaust pipe. Moreover, in the case where theaverage particle diameter of the inorganic glass particles is about 3 μmor less, at the time of firing, softened inorganic glass particles areless likely to fill in the concave portions on the surface of theexhaust pipe so that concave portions are less likely to be unfavorablyformed on the surface of the exhaust pipe.

Moreover, in the case where the average particle size of the inorganicglass particles is about 3 μm or less, at the time of firing, inorganicglass particles may be easily stabilized in the paint solution.

The average particle size of the inorganic glass particles is desirablyabout 0.1 μm or more. In the case where the average particle size of theinorganic glass particles is about 0.1 μm or more, the glass componentis less likely to be eluted in the paint so as not to adversely affectthe stability of the paint.

The paint preferably further contains the inorganic particles.

The average particle size of the inorganic particles and the averageparticle size of the inorganic glass particles are not particularlylimited, and one or both of them may be larger than about 3 μm; however,desirably, the average particle size of the inorganic particles is about3 μm or less, and the average particle size of the inorganic glassparticles is about 3 μm or less.

More desirably, the average particle size of the inorganic particles isabout 1 μm or less, and the average particle size of the inorganic glassparticles is about 1 μm or less. Furthermore desirably, the averageparticle size of the inorganic particles is about 0.9 μm or less, andthe average particle size of the inorganic glass particles is about 0.8μm or less.

In the case where both of the average particle sizes of the inorganicparticles and of the inorganic glass particles is about 3 μm or less,the sizes of particles are less likely to be varied. In this case,during the electrocoating process, since the flow of the coating film isless likely to be locally disturbed, paths are easily formed. As aresult, concave portions are easily formed on the surface of the exhaustpipe.

Moreover, in the case where the average particle size of the inorganicglass particles is about 3 μm or less, at the time of firing, softenedinorganic glass particles are less likely to fill in the concaveportions on the surface of the exhaust pipe so that concave portions areless likely to be unfavorably formed on the surface of the exhaust pipe.

Moreover, in the case where the average particle size of the inorganicparticles is about 3 μm or less, at the time of firing, solid inorganicparticles are dispersed in the inorganic glass particles softened toliquid form. In this case, when the inorganic particles are small,composite particles of the softened inorganic glass particles and theinorganic particles have high viscosity and poor flowability incomparison with the case where the inorganic particles are large, sothat the composite particles are less likely to fill in the concaveportions on the surface of the exhaust pipe. When the concave portionsare filled, the surface area of the surface coating layer becomes small,causing a reduction in emissivity.

Moreover, the average particle size of the inorganic particles isdesirably about 0.1 μm or more. The average particle size of theinorganic glass particles is desirably about 0.1 μm or more.

The average particle size of the inorganic particles and the averageparticle size of the inorganic glass particles can be measured by using,for example, a Shimadzu nano-particle size distribution measuringapparatus (SALD-7100) manufactured by SHIMADZU Corporation.

The amount of the inorganic glass particles blended is about 40% byweight as a desirable lower limit and about 99.5% by weight as adesirable upper limit relative to the total weight of the powder of theinorganic glass particles and the powder of the inorganic particles.

The inorganic glass particles correspond to a material that softens inthe firing treatment to form a matrix. The inorganic glass particles areformed into the inorganic glass base material in the firing treatment.

In the case where the amount of the inorganic glass particles blended isabout 40% by weight or more, since the amount of the inorganic glassparticles is not too small relative to the amount of the inorganicparticles, a matrix is easily formed sufficiently so that the inorganicglass particles easily fill in gaps among the inorganic particles, andthus a surface coating layer with few voids is likely to be formed. Thesurface coating layer with many voids causes a reduction in strength ofthe surface coating layer, failing to provide adhesion.

Moreover, in the case where the amount of the inorganic glass particlesblended is about 40% by weight or more, since the amount of theinorganic glass particles that are brought into contact with a metalbase material is not too small so that a reduction in the contact areabetween the softened inorganic glass particles and the metal basematerial during the firing process are less likely to be caused. As aresult, the surface coating layer is likely to be sufficiently bondedonto the metal base material. Consequently, at the time of firing orupon loading thermal impact, the surface coating layer is less likely toeasily drop off (to be separated).

On the other hand, when the amount of the inorganic glass particlesblended is about 99.5% by weight or less, the amount of the inorganicparticles is not too small, and the heat releasing characteristic of themanufactured exhaust pipe is less likely to deteriorate.

The amount of the inorganic glass particles blended is about 60% byweight as a more desirable lower limit and about 80% by weight as a moredesirable upper limit.

The paint may not contain the inorganic particles. Even in this case, asdescribed with reference to FIG. 7A to FIG. 7E, it is possible to obtainan exhaust pipe having an excellent heat releasing characteristic to acertain degree because of the electrocoating resin contained in thepaint.

In the case where the inorganic particles are contained in the paint,the amount of the inorganic particles blended is about 0.5% by weight asa desirable lower limit and about 60% by weight as a desirable upperlimit relative to the total weight of the powder of the inorganic glassparticles and the powder of the inorganic particles.

In the case where the amount of the inorganic particles blended is about0.5% by weight or more, since the amount of the inorganic particles isnot too small relative to the amount of the inorganic glass particles,the heat releasing characteristic of the exhaust pipe is less likely tobe deteriorated. On the other hand, in the case where the amount of theinorganic particles blended is about 60% by weight or less, the amountof the inorganic glass particles that contribute to bonding between thesurface coating layer and the metal base material is not too small. As aresult, the surface coating layer in the manufactured exhaust pipe isless likely to drops off.

The amount of the inorganic particles blended is about 20% by weight asa more desirable lower limit and about 40% by weight as a more desirableupper limit.

The electrocoating resin is desirably prepared as an anionicelectrocoating resin.

The anionic electrocoating resin has an anionic group. The anionic groupis a functional group that reacts with a base to form a salt. Examplesof the anionic group include, but are not particularly limited to, acarboxyl group, a sulfonic acid group, and a phosphoric acid group.

Moreover, examples of the anionic electrocoating resin include, but arenot particularly limited to, an acrylic resin, an epoxy resin, apolyurethane resin, a maleic resin, a polyester resin, and apolybutadiene resin.

Examples of the acrylic resin include, but are not particularly limitedto, copolymer acrylic resins obtained by polymerizing a monomercomposition including a carboxyl group-containing ethylene polymerizablemonomer and another ethylene polymerizable monomer.

Examples of the carboxyl group-containing ethylene polymerizable monomerinclude, but are not particularly limited to, (meth)acrylic acid, a(meth)acrylic acid dimer, crotonic acid, 2-(meth)acryloyloxyethylphthalic acid, 2-(meth)acryloyloxyethyl succinic acid,2-(meth)acryloyloxyethyl acid phosphate,2-(meth)acrylamide-2-methylpropane sulfonic acid,ω-carboxy-polycaprolactone mono(meth)acrylate, isocrotonic acid,α-hydro-ω-((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-1,6-hexanediyl), maleicacid, fumaric acid, itaconic acid, 2-vinylsalicylic acid, and3-vinylacetyl salicylic acid. These may be used alone, or two or morekinds of these may be used in combination.

Examples of the another ethylene polymerizable monomer include, but arenot particularly limited to, (meth)acrylates having an ester unit withone or more carbon atoms (for example, methyl(meth)acrylate,ethyl(meth)acrylate, n-propyl(meth)acrylate, isopropyl(meth)acrylate,n-butyl(meth)acrylate, isobutyl(meth)acrylate, t-butyl(meth)acrylate,2-ethylhexyl(meth)acrylate, lauryl(meth)acrylate, phenyl(meth)acrylate,isobornyl(meth)acrylate, cyclohexyl(meth)acrylate,t-butylcyclohexyl(meth)acrylate, dicyclopentadienyl(meth)acrylate,dihydrodicyclopentadienyl(meth)acrylate, etc.), polymerizable amidecompounds (for example, (meth)acrylamide, N-methylol(meth)acrylamide,N,N-dimethyl(meth)acrylamide, N,N-dibutyl(meth)acrylamide,N,N-dioctyl(meth)acrylamide, N-monobutyl(meth)acrylamide,N-monooctyl(meth)acrylamide, 2,4-dihydroxy-4′-vinylbenzophenone,N-(2-hydroxyethyl)(meth)acrylamide, etc.), polymerizable aromaticcompounds (for example, styrene, α-methyl styrene, t-butyl styrene,parachlorostyrene, vinyl naphthalene, etc.), polymerizable nitriles (forexample, (meth)acrylonitrile, etc.) α-olefins (for example, ethylene,propylene, etc.), vinyl esters (for example, vinyl acetate, vinylpropionate, etc.), dienes (for example, butadiene, isoprene, etc.),hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate,hydroxybutyl(meth)acrylate, allyl alcohol, (meth)acryl alcohol, andadducts of hydroxyethyl(meth)acrylate and ε-caprolactone. These may beused alone, or two or more of these may be used in combination.

In the case where two or more kinds of (meth)acrylates are used incombination as the (meth)acrylate having an ester unit with one or morecarbon atoms, (meth)acrylate having an ester unit with one or two carbonatoms is desirably included in the two or more kinds of (meth)acrylates.

In particular, when inorganic particles are contained in the paint, theelectrocoating resin is desirably prepared as an anionic electrocoatingresin.

Referring to FIG. 8, the following description will describe the reasonsfor this.

FIG. 8 is an explanatory drawing that illustrates a correspondingrelationship between a pH value and an electric charge with respect toinorganic glass particles and inorganic particles in accordance with theembodiment of the present invention.

As shown in FIG. 8, the equipotential point of the inorganic glassparticles is about a pH value of 2. Therefore, the inorganic glassparticles are charged positively under an environment having a pH valuesmaller than 2, while the inorganic particles are charged negativelyunder an environment having a pH value greater than 2.

Moreover, the equipotential point of the inorganic particles is about apH value of 7. Therefore, the inorganic particles are charged positivelyunder an environment having a pH value smaller than 7, while theinorganic particles are charged negatively under an environment having apH value greater than 7.

In other words, the electric charge of the inorganic glass particles isdifferent from that of the inorganic particles under an acidicenvironment of pH 2 to 7, while the electric charge of the inorganicglass particles is the same as that of the inorganic particles under analkaline environment.

Therefore, in order to allow both the inorganic glass particles and theinorganic particles to deposit simultaneously, the electrocoatingprocess is desirably carried out under an alkaline environment.

As described above, in the anionic electrocoating process, since a baseis used as a neutralizer for an anionic electrocoating resin, theelectrocoating process is carried out under an alkaline environment. Onthe other hand, in the cationic electrocoating process, since an acid isused as a neutralizer for a cationic electrocoating resin, theelectrocoating process is carried out under an acidic environment.

Therefore, the anionic electrocoating process carried out under analkaline environment is more desirable than the cationic electrocoatingprocess. That is, as the electrocoating resin contained in the paint,the anionic electrocoating resin is more desirably used than thecationic electrocoating resin.

As described above, the anionic electrocoating resin is more desirablyused as the electrocoating resin; however, the cationic electrocoatingresin may also be used. In the case of using the cationic electrocoatingresin, although the cationic electrocoating resin is inferior to theanionic electrocoating resin in stability of the paint and easiness informing concave portions, the cationic electrocoating resin is notunusable in the present embodiment.

The cationic electrocoating resin has a cationic group. The cationicgroup is a functional cationic group that reacts with an acid to formasalt. The cationic group is not particularly limited, and examplesthereof include an amino group, a sulfide group, and a phosphine group.

Moreover, examples of the cationic electrocoating resin include, but arenot particularly limited to, an acrylic resin, an epoxy resin, and apolyester resin.

In the case where the epoxy resin is an amino group-containing epoxyresin, the electrocoating resin is manufactured by opening of the epoxyring inside the raw epoxy resin molecule through a reaction with aminessuch as a primary amine, a secondary amine, and a tertiary amine.

Examples of the raw epoxy resin include, but are not particularlylimited to, polyphenol polyglycidyl ether-type epoxy resins that arereaction products of polycyclic phenolic compounds and epichlorohydrin,such as a bisphenol A-type epoxy resin, a bisphenol F-type epoxy resin,a bisphenol S-type epoxy resin, phenol novolac, and cresol novolac, andoxazolidone ring-containing epoxy resins that are obtained by reactionof epichlorohydrin and a diisocyanate compound or a bisurethane compoundobtained by blocking an NCO group of a diisocyanate compound by a loweralcohol such as methanol, ethanol or the like.

Examples of the amines include, but are not particularly limited to,butyl amine, octyl amine, diethyl amine, dibutyl amine, methylbutylamine, monoethanol amine, diethanol amine, N-methylethanol amine, atriethyl amine acid salt, an N,N-dimethylethanol amine acid salt, andketimine-blocked amino group-containing polyamine.

The ketimine-blocked amino group-containing polyamine refers to an aminein which an amino group is blocked by ketimine. Examples of theketimine-blocked amino group-containing polyamine include those obtainedby converting an amino group in polyamines such as amino ethylethanolamine, diethylene triamine, dipropylene triamine, dibutylene triamine,and triethylene tetramine, to ketimine by reaction with ketones such asacetone, methylethylketone, and methylisobutylketone (for example,ketimine-blocked primary amino group-containing secondary amines, suchas diethylene triamine methylisobutyl ketimine and aminoethylethanolamine methylisobutyl ketimine).

Examples of the acrylic resins include, but are not particularly limitedto, those resins obtained by allowing an amine to react with theoxysilane ring of the resultant product obtained bycopolymerizing(meth)acrylate, hydroxyl group-containing (meth)acrylatemonomers (for example, adducts of hydroxyl group-containing(meth)acrylesters, such as 2-hydroxyethyl(meth)acrylate,2-hydroxypropyl(meth)acrylate and 2-hydroxyethyl(meth)acrylate, andε-caprolactone), and other acrylic and/or non-acrylic monomers, andthose resins obtained by copolymerizing an acrylic monomer having anamino group without using glycidyl(meth)acrylate.

As the electrocoating resin, only one kind of the electrocoating resinmay be used, or a plurality of kinds of electrocoating resins may beused.

Moreover, with respect to the paint, an organic binder material maycontain only an electrocoating resin or also contain an organic bindermaterial other than the electrocoating resin. Examples of the organicbinder material other than the electrocoating resin include polyvinylalcohol, methylcellulose and ethylcellulose. These may be used alone ortwo or more kinds of these may be used in combination.

The T_(g)s of the plurality of kinds of electrocoating resins aredesirably different from one another.

During the electrocoating process, the flowability of the electrocoatingresin is changed at temperatures in the vicinity of the T_(g), and inthe case where the T_(g)s of the plurality of kinds of electrocoatingresins are different from one another, the abrupt change in theflowability of the electrocoating resin may be easily suppressed. Whenthe T_(g)s of the plurality of kinds of electrocoating resins aredifferent from one another, the paint is less vulnerable to influencesfrom the temperature during the application thereof, facilitating stablemanufacturing of a coat film in a comparatively wide temperature range.Consequently, the temperature dependence of the paint is easilyalleviated during the application thereof. As a result, an exhaust pipehaving a surface coating layer with desired concave portions formed onthe surface thereof is likely to be provided.

Moreover, the T_(g) of the electrocoating resin is desirably from about5° C. to about 50° C.

In the case where the T_(g) of the electrocoating resin is about 5° C.or higher, the viscosity as a coat film is not lowered, and the resin isless likely to be flowable. Consequently, during the electrocoatingprocess, once formed on the coat film, paths are less likely to befilled with the coat film because the peripheral electrocoating resin isless likely to be softened and flown. For this reason, preferableconcave portions are likely to be formed, and a surface coating layerhaving high emissivity is likely to be obtained. On the other hand, whenthe T_(g) of the electrocoating resin is about 50° C. or lower, theelectrocoating resin is not too hard at room temperature and is noteasily flown. As a result, desired paths are likely to be formed on thecoat film. For this reason, preferable concave portions are likely to beformed, and a surface coating layer having high emissivity is easilyobtained. Moreover, in the case where the T_(g) of the electrocoatingresin is about 50° C. or lower, since the flowability of theelectrocoating resin is not deteriorated, inner moisture is easilyevaporated during drying and curing treatment, and extra time is notrequired for the drying and curing treatment. For this reason, a workingefficiency is improved to decrease costs.

However, the T_(g) of the electrocoating resin may fall outside therange from about 5° C. to about 50° C. In the case of using theplurality of kinds of electrocoating resins, an electrocoating resinhaving T_(g) falling out of the range from about 5° C. to about 50° C.may be included in the plurality of kinds of electrocoating resins, orall T_(g)s of the electrocoating resins may fall outside the range fromabout 5° C. to about 50° C.

Additionally, T_(g) refers to a glass transition point, and can bemeasured by a DSC (Differential Scanning calorimeter) according to JIS K7121: 1987.

The weight ratio of the electrocoating resin relative to the totalweight of the inorganic particles and the inorganic glass particles isdesirably from about 1.0 to about 3.5.

In the case where no inorganic particles are contained in the paint, theweight ratio of the electrocoating resin relative to the weight of theinorganic glass particles is desirably from about 1.0 to about 3.5.

In the case where the weight ratio of the electrocoating resin relativeto the total weight of the inorganic particles and the inorganic glassparticles is about 3.5 or less (including the case where the weight ofthe inorganic particles is 0), since the amount of the electrocoatingresin contained in the paint is not too large, the volume ratio of theinorganic particles and inorganic glass particles becomes not too low,so that the inorganic particles and the inorganic glass particles areless likely to be separated from one another in the coat film.Consequently, the inorganic particles and the inorganic glass particlesare likely to be combined to each other; therefore, upon degreasing theelectrocoating resin, even when the electrocoating resin is heated to beburned out, the inorganic particles and the inorganic glass particlesare less likely to collapse to easily drop off. As a result, a surfacecoating layer having high emissivity is likely to be obtained.

On the other hand, in the case where the weight ratio of theelectrocoating resin relative to the total weight of the inorganicparticles and the inorganic glass particles is about 1.0 or more(including the case where the weight of the inorganic particles is 0),since the amount of the electrocoating resin is not too small, thedensities of the inorganic particles and inorganic glass particlescontained in the paint is not too high, and the ratio of solid-statecomponents (particles) in the coat film deposited in the electrocoatingprocess is also not too high. Consequently, since the flowability of thecoat film during application of the electrocoating current is good, thepath formation and coat film formation on the periphery of the pathseasily progress. As a result, desired concave portions are also easilyformed on the surface of the exhaust pipe. As a result, a surfacecoating layer having high emissivity is likely to be obtained. Moreover,when the amounts of the inorganic particles and inorganic glassparticles contained in the paint are not too great, the precipitation ofthe inorganic particles and the inorganic glass particles hardly occurs,and the particle concentration in the coating solution hardly changes.Then, fluctuations in the coating conditions are less likely to becaused. As a result, stable formation of the coat film is likely to berealized. Moreover, another problem is less likely to be raised that theparticles sediment onto the bottom surface of the electrocoating vessel.However, the weight ratio of the electrocoating resin relative to thetotal amount of the inorganic particles and the inorganic glassparticles may fall outside the range from about 1.0 to about 3.5.Moreover, in the case where no inorganic particles are contained in thepaint, the weight ratio of the electrocoating resin relative to theweight of the inorganic glass particles may fall outside the range fromabout 1.0 to about 3.5.

In addition to the inorganic glass particles, the inorganic particles,and the organic binder material, the paint may further contain apigment, a neutralizer, a curing agent, a dispersion medium, and variousother additives.

Examples of the pigment include a colorant pigment, an extender pigment,and a rust-proofing pigment.

Examples of the colorant pigment include Titanium White, carbon black,iron oxide red, Phthalocyanine Blue, Phthalocyanine Green, monoazoyellow, disazo yellow, Benzimidazolone Yellow, Quinacridone Red, monoazored, polyazo red, and Perylene Red.

Examples of the extender pigment include kaolin, talc, aluminumsilicate, calcium carbonate, mica, clay, and silica.

Examples of the rust-proofing pigment include zinc phosphate, ironphosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinccyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminummolybdate, calcium molybdate, aluminum phosphomolybdate, and aluminumzinc phosphomolybdate.

Examples of the neutralizer for the anionic electrocoating resin includebases, such as ammonia, organic amine, and alkali metal hydroxide.

Examples of the organic amine include diethyl amine, ethylethanol amine,diethanol amine, monoethanol amine, monopropanol amine, isopropanolamine, ethylaminoethyl amine, hydroxyethyl amine, and diethylenetriamine.

Examples of the alkali metal hydroxide include sodium hydroxide andpotassium hydroxide.

Examples of the neutralizer for the cationic electrocoating resininclude acids, such as hydrochloric acid, nitric acid, phosphoric acid,formic acid, acetic acid, and lactic acid.

Examples of the curing agent for the anionic electrocoating resininclude a melamine resin and block polyisocyanate.

Examples of the curing agent for the cationic electrocoating resininclude block polyisocyanate.

The block polyisocyanate is obtained by blocking polyisocyanate using ablocking agent.

Polyisocyanate refers to a compound having two or more isocyanate groupsin one molecule.

Examples of the dispersion medium include water, or organic solventssuch as methanol, ethanol and acetone.

Examples of the various other additives include an additive blended forcarrying out the electrocoating process, a surfactant, an antioxidant,and an ultraviolet ray absorbing agent.

Examples of the additive blended for carrying out the electrocoatingprocess include an additive for use in controlling a zeta potentialand/or adjusting the resistance value of the solution, and a stabilizerfor use in ensuring the dispersibility of the inorganic glass particlesand/or inorganic particles.

The above description has discussed the paint in accordance with theembodiment of the present invention.

In the embodiment of the present invention, the exhaust pipe accordingto the embodiment of the present invention is manufactured using thepaint.

The following description will discuss a method for manufacturing theexhaust pipe according to the embodiment of the present embodiment.

Additionally, the following description will discuss the case whereinorganic particles are contained in the paint.

(1) Manufacturing of Paint

The above inorganic glass particles and inorganic particles aredry-mixed to give a mixed powder.

More specifically, powder of the inorganic glass particles and powder ofthe inorganic particles are prepared so as to have predeterminedparticle size and shape, respectively. Then, the both powders are mixedat a predetermined blending ratio to give a mixed powder.

To the mixed powder thus prepared, the electrocoating resin and variousadditives such as water are added and mixed with one another. In thismanner, the paint is manufactured.

(2) Preparation of Base Material

Abase material made of a metal (metal base material) is used as astarting material. First, the metal base material is subjected to awashing treatment so that impurities are removed from the surfacethereof.

The washing treatment is not particularly limited, and a conventionallyknown method may be used. For example, an ultrasonic washing process inan alcohol solvent may be used.

After the washing treatment, if necessary, the surface of the basematerial may be subjected to a roughening treatment for increasing thespecific surface area of the base material or adjusting the roughness ofthe surface of the base material. For example, roughening treatments,such as a sandblasting treatment, an etching treatment and ahigh-temperature oxidizing treatment, may be carried out. Thesetreatments may be carried out alone, or two or more of these may becarried out in combination.

(3) Formation of Coat Film

The paint manufactured in the treatment of (1) is applied to the surfaceof the metal base material prepared in the treatment of (2) byelectrocoating. More specifically, the metal base material and anelectrode plate are placed in the paint. Then, a voltage is appliedthereto, wherein one of the metal base material and the electrode platesfunctions as an anode and the other functions as a cathode.

Then, the electrocoating resin in the soluble form carries the inorganicglass particles and the inorganic particles to the surface of the metalbase material (see FIG. 7A). When brought into contact with the surfaceof the metal base material, the electrocoating resin changes from thesoluble form to the insoluble form, and is deposited on the surface ofthe metal base material (see FIG. 7B and FIG. 7C). At this time, pathsthrough which the soluble electrocoating resin is allowed to pass areformed in the coat film (see FIG. 7B and FIG. 7C). These paths areformed when the oxygen gas, which is generated when the electrocoatingresin is brought into contact with the surface of the metal basematerial, thrusts the coat film that has already been formed aside toget into the coating solution. After the electrocoating, theelectrocoating resin may remain inside the paths. Since theelectrocoating resin is not deposited on the surface of the metal basematerial and is soluble, the resin may be washed away with water.

The electrocoating process is desirably carried out normally by applyinga voltage of from about 50 V to about 450 V at a bath temperature offrom about 10° C. to about 45° C. for a time period of from about 15seconds to about 20 minutes, and the voltage is more desirably fromabout 60 V to about 300 V, the bath temperature is more desirably fromabout 26° C. to about 32° C., and a current-applying time is moredesirably from about 30 seconds to about 10 minutes. Moreover, thesolids concentration of the paint is desirably from about 5% by weightto about 25% by weight, and the pH of the paint is desirably from about8.0 to about 9.5.

In the case where the voltage is about 300 V or less, a coat film of thepaint that has been once formed on the surface of a body to be coated(metal base material) is less likely to be dissolved again due to heatgenerated on the surface of the body to be coated. As a result, the filmthickness of the coat film is less likely not to grow thicker in spiteof voltage application. On the other hand, in the case where the voltageis about 60 V or more, since the load voltage is not too low, the forceattracting the electrocoating resin onto the body to be coated (metalbase material) is less likely to be weak, so that a sufficiently thickcoat film of the paint is likely to be obtained. As a result, a surfacecoating layer having high emissivity is likely to be obtained.

When the bath temperature is about 32° C. or lower, the electrocoatingresin in the coating solution is less likely to deteriorate due to heat.As a result, since the frequency of replacement of the electrocoatingresin due to deterioration of the paint does not increase, themanufacturing cost is decreased. On the other hand, in the case wherethe bath temperature is about 26° C. or higher, since the activity ofthe electrocoating resin is not lowered and the reaction rate on thesurface of the body to be coated (metal base material) is not reduced, acoat film of the paint is likely to be obtained. As a result, a surfacecoating layer having high emissivity is likely to be obtained.

In the case where the current-applying time is about 10 minutes or less,since the current-applying time is not too long, the film thicknesses ofthe coat film formed of the paint is less likely to be different betweenthe perpendicular surface and horizontal surface of the body to becoated (metal base material) because the solid components of the paintare less likely to be segmented. As a result, a surface coating layerhaving a uniform heat releasing characteristic over the entire surfaceis likely to be obtained. On the other hand, in the case where thecurrent-applying time is about 30 seconds or more, since thecurrent-applying time is not too short, the growth of the coat film isnot stopped halfway, so that a sufficiently thick coat film formed ofthe paint is likely to be obtained. As a result, a surface coating layerhaving high emissivity is likely to be obtained.

In the case where the solids concentration of the paint is about 25% byweight or less, since the flowability of the coat film deposited byelectrocoating is not lowered, so that heat and bubbles, which aregenerated on the surface of the body to be coated (metal base material),are easily removed. Consequently, a local temperature rise is lesslikely to cause a case where the coat film tends to be dissolved againin the coating solution or bubbles remaining in the coat film arethermally expanded upon heating to cause bumping, so that the surfacecondition of the coat film formed of the paint deteriorates. As aresult, a surface coating layer having high emissivity is likely to beobtained. On the other hand, in the case where the solids concentrationof the paint is about 5% by weight or more, not only the electrocoatingresin is deposited on the body to be coated (metal base material), butalso certain amounts of the inorganic glass particles and the inorganicparticles are attached to the body to be coated (metal base material).In such a case, the coat film of the paint is easily formed.Consequently, the surface coating layer that remains on the metal basematerial after firing process is less likely to be thin. As a result, asurface coating layer having high emissivity is likely to be obtained.

In the case where the pH of the paint is about 9.5 or less, the coatfilm of the paint is easily deposited and power consumption required forelectrocoating of the paint is less likely to increase. As a result,energy consumption is less likely to increase. This is presumably causedby the fact that, as a mechanism of the formation of a coat film in theelectrocoating process, since the pH changes due to an electric reactionon the surface of the body to be coated (metal base material), theelectrocoating resin is changed from the soluble form to the insolubleform to be deposited. In the case where the pH of the paint is about 9.5or less, since the pH is not uselessly too high, the electrocoatingresin is less likely to be kept in the soluble form not to be depositedand the coat film once deposited is less likely to be dissolved again inthe coating solution when the pH is lowered by an electric reaction.Moreover, in the case where the pH of the paint is about 9.5 or less,the coat film state of the paint is less likely to be hardly stabilizeddue to bubbling. As a result, a large number of voids are less likely tobe generated in the coat film, the strength of the surface coating layeris less likely to be lowered, so that the high adhesion is likely to beobtained.

On the other hand, in the case where the pH of the paint is about 8.0 ormore, the pH value is not in the vicinity of pH at which theelectrocoating resin itself changes from the insoluble form to thesoluble form so that the state of the electrocoating resin is not on aborder between the soluble state and the insoluble state. In such acase, the electrocoating resin does not change its form between thesoluble form and the insoluble form along with the fluctuations of thepH. Accordingly, the electrocoating resin is less likely to unstablyexist in the solution. As a result, since the frequency of replacementof the electrocoating resin due to deterioration of the paint does notincrease, so that the manufacturing cost is lowered.

Additionally, in the case of using an anionic electrocoating resin asthe electrocoating resin, the metal base material is allowed to functionas an anode, and the electrode plate is allowed to function as acathode. On the other hand, in the case of using a cationicelectrocoating resin as the electrocoating resin, the metal basematerial is allowed to function as a cathode, and the electrode plate isallowed to function as an anode; thus, a voltage is applied thereto.

(4) Drying and Curing

The metal base material to which the paint has been applied in thetreatment of (3) is heated to a predetermined temperature so that thecoat film of the paint formed on the surface of the metal base materialis dried and cured. At this time, along with evaporation of moisture,volatile resin additives and the like through the paths formed in thetreatment of (3), preliminary portions for concave portions are formedon the surface of the coat film (see FIG. 7D). In this case, thepreliminary portions for concave portions refer to concave areas formedon the surface of the coat film, and the areas correspond to areasgenerated by deformed paths caused by drying and curing of the coatfilm.

In the present treatment, the heating temperature is desirably fromabout 100° C. to about 200° C., more desirably from about 110° C. toabout 190° C., and furthermore desirably from about 120° C. to about180° C.

In the case where the heating temperature is about 200° C. or lower,since the temperature is not too high, the coat film of the paint isless likely to be cured too much. As a result, wasteful energyconsumption is not likely to be caused. On the other hand, in the casewhere the heating temperature is about 100° C. or higher, the drying andcuring is less likely to become insufficient, and moisture or thesolvent is less likely to remain in the coat film. As a result, uponheating in the degreasing or firing treatment, the residual moisture orsolvent is less likely to cause bumping to cause rupturing of the coatfilm. As a result, partially unattached portions (portions where voidsare present in the surface coating layer) are less likely to be formed.Moreover, since the coat film of the paint is likely to be sufficientlycured, the adhesion between the coat film and the metal base material isless likely to be lowered so that separation is less likely to occurupon handling.

Moreover, the heating temperature is preferably maintained for apredetermined period of time, and the retention time is desirably fromabout 5 minutes to about 90 minutes.

When the retention time is about 90 minutes or less, the coat film ofthe paint is less likely to be cured too much so that a waste of time isless likely to be caused. On the other hand, when the retention time isabout 5 minutes or more, the drying and curing of the coat film of thepaint is less likely to be insufficient, so that moisture or the solventis less likely to remain in the coat film. As a result, upon heating inthe degreasing or firing treatment, the residual moisture or solvent isless likely to cause bumping to cause rupturing of the surface coatinglayer. As a result, partially unattached portions (portions where voidsare present in the surface coating layer) are less likely to be formed.Moreover, since the coat film of the paint is likely to be sufficientlycured, the adhesion between the coat film and the metal base material isless likely to be lowered, so that separation is less likely to occurupon handling.

(5) Degreasing After the treatment of (4), the metal base material isheated to a temperature not lower than the burning-out temperature ofthe electrocoating resin so that the electrocoating resin is burned out.Thus, the volume of the coat film is contracted so that concave portionsare easily formed on the surface of the coat film based on thepreliminary portions for concave portions on the surface of the coatfilm formed in the treatment of (4) (see FIG. 7E).

The burning-out temperature of the electrocoating resin refers to atemperature at which the weight of the electrocoating resin is reducedby about 50%, and this temperature can be measured by a TG/DTAsimultaneous thermal analyzer.

Although it also depends on the kind of the blended electrocoatingresin, the heating temperature in this treatment is desirably from about300° C. to about 600° C., more desirably from about 325° C. to about550° C., and furthermore desirably from about 350° C. to about 500° C.

In the case where the heating temperature is about 600° C. or lower,since the temperature is not too high, the inorganic glass particles areless likely to be softened before completion of the degreasing of theelectrocoating resin. As a result, after the inorganic glass particleshave softened to start the formation of a matrix, the electrocoatingresin is less likely to be burned to generate gas to cause bumping. As aresult, unattached portions are less likely to be formed. On the otherhand, in the case where the heating temperature is about 300° C. orhigher, the electrocoating resin is less likely to be insufficientlydegreased, so that resin components are less likely to remain in thecoat film. For this reason, upon a temperature rise in the followingfiring treatment of (6), bumping is less likely to be caused so thatunattached portions are less likely to be formed.

In the present treatment, the heating temperature is maintained for apredetermined period of time. The retention time is desirably from about5 minutes to about 90 minutes.

Since the retention time of about 90 minutes is enough for completion ofthe degreasing of the electrocoating resin, the retention time of about90 minutes or less is less likely to cause a waste of time. On the otherhand, in the case where the retention time is about 5 minutes or more,the degreasing of the electrocoating resin is less likely to becomeinsufficient, so that resin components are less likely to remain in thecoat film. For this reason, upon a temperature rise in the firingtreatment of (6), the electrocoating resin is less likely to be burnedin the softened inorganic glass particles to generate a gas to causebumping. Accordingly, holes through which the metal base material isexposed are less likely to be formed on the surface of the surfacecoating layer. As a result, a surface coating layer having highemissivity is likely to be obtained.

Moreover, the rate of the temperature rise from the heating temperaturein the drying and curing treatment of (4) to the heating temperature inthe present treatment is desirably from about 1.7° C./minute to about60.0° C./minute, more desirably from about 2.0° C./minute to about 30.0°C./minute, and furthermore desirably from about 3.0° C./minute to about15.0° C./minute.

In the case where the rate of the temperature rise is about 60.0°C./minute or less, bumping of the resin component is less likely tooccur, so that unattached portions are less likely to be formed. On theother hand, in the case where the rate of the temperature rise is about1.7° C./minute or more, too much time is not required for thetemperature rise, so that a waste of time is less likely to be caused.

As described above, in the degreasing treatment of (5), the temperatureis maintained at a predetermined temperature for a predetermined periodof time. In this manner, in the degreasing treatment of (5),“maintenance at a predetermined temperature for a predetermined period”is an essential condition.

In the present embodiment, the degreasing treatment of (5) is desirablycarried out; however, the firing treatment of (6) may be carried outwithout preliminary carrying out the degreasing treatment of (5).

The following description will be given on the premise that thedegreasing treatment of (5) is preliminarily carried out.

(6) Firing

After the treatment of (5), the metal base material is heated to atemperature not lower than the softening point of the inorganic glassparticles.

With this arrangement, the metal base material is likely to firmlyadhere to the inorganic glass particles so that a surface coating layerthat firmly adhere to the metal base material is easily formed.Moreover, since the inorganic glass particles are fused, the reductionrate of the film thickness of the coat film becomes great so that asurface coating layer with concave portions on its surface is easilyformed (see FIG. 7E).

Furthermore, when the inorganic particles are present in the coat film,since the flowability of the softened inorganic glass particles islowered, the flow of the coat film is suppressed on the periphery ofconcave portions. Then, a surface coating layer with concave portions onits surface is easily formed. In contrast, in the case where noinorganic particles are present in the coat film, since the flow of thecoat film is not suppressed on the periphery of concave portions, it ismore difficult to form a surface coating layer with concave portions onits surface in comparison with the case where the inorganic particlesare present in the coat film.

The surface coating layer herein refers to a layer formed on the surfaceof the metal base material and is formed by firing of the coat film.

Although it also depends on the kind of the blended inorganic glassparticles, the heating temperature in the present treatment is desirablyfrom about 500° C. to about 1000° C., more desirably from about 600° C.to about 950° C., and furthermore desirably from about 700° C. to about900° C.

In the case where the heating temperature is about 1000° C. or lower,the metal base material is less likely to deteriorate due to exposure tothe high temperature. On the other hand, in the case of the heatingtemperature of about 500° C. or higher, the softening of the inorganicglass particles is less likely to be insufficient, so that insufficientsintering is less likely to be caused. Consequently, the coating of thepaint is likely to be densified, so that sufficient adhesion is likelyto be obtained between the metal base material and the surface coatinglayer.

Additionally, the heating temperature of the present treatment is higherthan the heating temperature in the degreasing treatment of (5).

Moreover, the temperature is desirably maintained at the heatingtemperature for a predetermined period of time, and the retention timeis desirably in a range from about 1 minute to about 30 minutes.

When the retention time is about 30 minutes or less, the metal basematerial is less likely to deteriorate. On the other hand, when theretention time is about 1 minute or more, the softening of the inorganicglass particles is less likely to be insufficient, so that the coatingis likely to be densified. As a result, sufficient adhesion is likely tobe obtained between the metal base material and the surface coatinglayer.

Moreover, the rate of the temperature rise from the heating temperatureduring the degreasing treatment of (5) to the heating temperature in thepresent treatment is desirably from about 3.3° C./minute to about 100.0°C./minute, more desirably from about 4.0° C./minute to about 50.0°C./minute, and furthermore desirably from about 5.0° C./minute to about25.0° C./minute.

In the case where the rate of the temperature rise is about 100.0°C./min or less, though the heat capacity of the metal base material isgreat and absorbs heat, the entire metal base material is likely to beevenly heated. On the other hand, in the case where the rate of thetemperature rise is about 3.3° C./minute or more, the time required forthe temperature rise is not too long, so that a waste of the time isless likely to be caused.

By carrying out the above-mentioned process, the exhaust pipe accordingto the embodiment of the present invention having a surface coatinglayer formed on a metal base material is manufactured.

The description has been given on the exhaust pipe according to theembodiment of the present invention.

Now, effects of the exhaust pipe according to the embodiment of thepresent invention are listed.

(1) In the exhaust pipe according to the embodiment of the presentinvention, concave portions and convex portions are formed on thesurface of the surface coating layer. Therefore, the surface area of theexhaust pipe becomes great so that the exhaust pipe is likely to havehigh apparent emissivity. Therefore, by accelerating the radiant heattransfer, the resultant exhaust pipe is likely to have a superior heatreleasing characteristic.

Moreover, the concave portions formed on the surface of the surfacecoating layer are likely to serve as a number of non-fixed ends fordispersing thermal stress. Furthermore, by the concave portions formedon the surface of the surface coating layer, portions having smallerfilm thicknesses are formed in the surface coating layer. Since atemperature difference in each of these portions becomes small in thethickness direction, thermal stress hardly occurs inside the surfacecoating layer. Therefore, the thermal stress due to thermal impact iseasily relieved to easily prevent separation of the surface coatinglayer. As a result, the exhaust pipe of the embodiment of the presentinvention is likely to easily maintain a high heat releasingcharacteristic.

(2) In the exhaust pipe according to the present embodiment, the convexportion is higher than a second reference surface having a height of(H_(max)−H×⅓), wherein H_(max) represents a maximum value of the heightof the surface coating layer, H_(min) represents a minimum value of theheight of the surface coating layer, and H represents a differencebetween H_(max) and H_(min).

In the exhaust pipe, convex portions higher than the second referencesurface are present on the peripheral edge portion. The second referencesurface is a face having a height of (H_(max)−H×⅓). Namely, in theexhaust pipe, an area much higher than the concave portion is present onthe peripheral edge portion of the concave portion (i.e. comparativelyin the vicinity of the concave portion). Therefore, the surface of thesurface coating layer forms a steep slope from the concave portion tothe convex portion. This increases the surface area to increase theapparent emissivity. This increase is likely to improve the heatreleasing characteristic.

(3) In the exhaust pipe according to the present embodiment, the concaveportion has a virtually circular shape when seen in a directionperpendicular to the surface of the metal base material.

This presumably allows the thermal stress due to thermal impact to beeasily relieved. In the case where separation has once occurred at anedge portion of the concave portion, if the concave portion has a linearshape, the separation proceeds successively; however, in the case wherethe concave portion has a virtually circular shape, the separation isless likely to proceed because the separated surface coating layer ispulled by the peripheral surface coating layer therearound. As a result,the adhesion between the surface coating layer and the metal basematerial is likely to be improved.

(4) In the exhaust pipe according to the present embodiment, d>0 ispreferably satisfied, wherein H_(min) represents a minimum value of theheight of the surface coating layer and d represents a distance betweena face having a height of H_(min) and the surface of the metal basematerial.

In the case of d>0, namely, in the case where the metal base material isnot exposed on the surface of the exhaust pipe, the effect of improvingthe emissivity by the concave portion formed on the surface of thesurface coating layer is likely to be achieved sufficiently. Moreover,since the metal base material has low emissivity, the effect ofimproving the emissivity is presumably less likely to be reduced. In thecase of d>0, the metal base material having low emissivity is notexposed on the surface, and therefore, deterioration in the heatreleasing characteristic is easily avoided.

(5) In the exhaust pipe according to the present embodiment, d≧about 2μm is satisfied.

A predetermined distance or more between the surface of the metal basematerial having low emissivity and the bottom of the concave portion islikely to provide a sufficient effect of improving the emissivity by theconcave portion formed on the surface of the surface coating layer, sothat high emissivity is likely to be achieved. Accordingly,deterioration in the heat releasing characteristic is likely to be moreeffectively avoided.

(6) In the exhaust pipe according to claim 6, the concave portion has avirtually circular shape having a diameter of from about 3 μm to about2000 μm when seen in a direction perpendicular to the surface of themetal base material.

As mentioned above, given that an increase in surface area of thesurface coating layer contributes to an improvement in emissivity,desirably, the size of the concave portion is small and the densitythereof is high.

However, in the case where the size of the concave portion is too small,the walls of the concave portion are made face to face with each otherclosely. In such a case, infrared rays radiated upon heating of thesurface coating layer are hardly radiated outside of the surface coatinglayer, resulting in low heat releasing effect. On the other hand, sincethe emissivity at the concave portion is low corresponding to the smallthickness of the surface coating layer, the emissivity of the entiresurface coating layer is lowered when the size of the concave portion istoo large, leading to a case where a high heat releasing characteristicis less likely to be obtained.

In the exhaust pipe according to the present embodiment, since theconcave portion has an appropriate size, the exhaust pipe is likely tohave an excellent heat releasing characteristic.

(7) In the exhaust pipe according to the present embodiment, the densityof the concave portions is desirably from about 10 pics/cm² to about 10⁷pics/cm².

Given that an increase in surface area of the surface coating layercontributes to an improvement in emissivity, the density of the concaveportions is desirably high. In the case where the density of the concaveportions is too low, since an increase in surface area is small, theeffect for improving the emissivity is hardly obtained.

On the other hand, in the case where the density of the concave portionsis too high, two concave portions are positioned too close to each otherso that they may be partially overlapped with each other. When the twoconcave portions are overlapped with each other, a convex part is formedbetween the two concave portions. Since this convex part is lower thanthe first reference surface, this convex part is not the aforementionedconvex portion, and is not continuously formed in a manner ofsurrounding the concave portion. Consequently, the convex part tends tobe a portion that is easily separated. For this reason, separationoccurs from the convex part as a starting point with an elapse of time,and the emissivity may possibly be lowered.

In the exhaust pipe according to the present embodiment, since concaveportions are formed at an appropriate density, the exhaust pipe islikely to have an excellent heat releasing characteristic.

(8) In the exhaust pipe according to the present embodiment, the surfacecoating layer further contains inorganic particles.

Since the inorganic particles are highly emissive, infrared rays arereleased strongly upon heating. This is indicated by Stefan-Boltzmannlaw represented by the following equation (1):q=εσ(T ₁ ⁴ −T ₂ ⁴)  (4)(σ: Stefan-Boltzmann constant . . . 5.67×10⁻⁸ [w/m²·K⁴], q: heat flux[W/m²], s: emissivity, T₁: heating unit temperature [K], T₂: heatreceiving unit temperature [K]).

Therefore, in an exhaust pipe containing inorganic particles in thesurface coating layer, infrared rays are emitted from the inorganicparticles in the surface coating layer. Then, the emissivity of thesurface coating layer becomes high so that such an exhaust pipe islikely to have an excellent heat releasing characteristic at hightemperature.

(9) In the exhaust pipe according to the present embodiment, theinorganic particles have an average particle size of not more than about3 μm.

As mentioned above, the inorganic particles have a function forimproving emissivity. For this reason, in the case where portions wherethe inorganic particles are present are projected onto a plane inparallel with the surface of the metal base material, the emissivitybecomes greater along with the increase in the area of the projectedportions.

If the average particle size of the inorganic particles is great, theinorganic particles are localized in some areas, while the other areaslack the inorganic particles. In this case, the above area is small.Consequently, the emissivity is lowered.

That is, in the case where the ratio of the inorganic particlescontained in the surface coating layer is constant, the area becomeslarger along with the reduction in the average particle size of theinorganic particles.

In the exhaust pipe according to the present embodiment, since theinorganic particles having an average particle size of not more thanabout 3 μm are used, the exhaust pipe is likely to have an excellentheat releasing characteristic at high temperature.

(10) In the exhaust pipe according to the present embodiment, theinorganic particles have an average interparticle distance of not morethan about 3 μm.

As mentioned above, the inorganic particles have a function forimproving emissivity. For this reason, in the case where portions wherethe inorganic particles are present are projected onto a plane inparallel with the surface of the metal base material, the emissivitybecomes greater along with the increase in the area of the projectedportions.

If the interparticle distance of the inorganic particles is great, theinorganic particles are localized in some areas, while the other areaslack the inorganic particles. In this case, the above area is small.Consequently, the emissivity is lowered.

That is, in the case where the ratio of the inorganic particlescontained in the surface coating layer is constant, the area becomeslarger along with the reduction in the interparticle distance of theinorganic particles.

In the exhaust pipe according to the present embodiment, since theaverage interparticle distance of the inorganic particles is small asabout 3 μm, the exhaust pipe is likely to have a superior heat releasingcharacteristic at high temperature.

EXAMPLES

The following description will discuss the present invention in detailby means of examples; however, the present invention is not limited tothese examples.

Example 1

(1) Manufacturing of Paint

As powder of inorganic particles, a powder of a metal oxide composed ofMnO₂ powder (24 parts by weight), FeO powder (8 parts by weight), CuOpowder (4 parts by weight) and CoO powder (4 parts by weight) wasprepared. The inorganic particles had an average particle size of 0.8μm.

Moreover, as a powder of inorganic glass particles, K807 (60 parts byweight) (SiO₂—BaO—B₂O₃ glass powder, softening point: 720° C.),manufactured by Asahi Glass Co., Ltd. was prepared. The inorganic glassparticles had an average particle size of 0.9 μm.

The powder of the inorganic particles and the powder of the inorganicglass particles were dry-mixed to prepare a mixed powder.

Moreover, to a reaction container were added a monomer composition, asolvent and a polymerization initiator. Then, the monomer compositionwas polymerized so that an anionic electrocoating resin wasmanufactured. More specifically, as the monomer composition, ethylacrylate (13 parts by weight), 2-ethylhexyl methacrylate (30 parts byweight), methyl methacrylate (31 parts by weight), acrylic acid (9 partsby weight), 2-hydroxyethyl acrylate (17 parts by weight) and N-methylolmethacryl amide (4 parts by weight) were added. As the solvent,isopropyl alcohol (IPA) (54 parts by weight) and butyl cellosolve (15parts by weight) were added thereto. As the polymerization initiator,azobis isobutylonitrile (3 parts by weight) was added thereto.

To the mixed powder, the anionic electrocoating resin (170 parts byweight), obtained by the polymerization, was added and mixed as anorganic binder.

Thereafter, pure water (1500 parts by weight) and other variousadditives were added and mixed so that a paint was manufactured.

The solids concentration of the paint thus manufactured was 15% byweight.

Measurement using a DSC (differential scanning calorimeter) (EXSTARDSC6220, manufactured by SII-Nanotechnology Inc.) clarified that T_(g)of the anionic electrocoating resin was 25° C.

(2) Preparation of Metal Base Material

As a metal base material, a plate-shaped stainless base material (madeof SUS430) having a width of 100 mm, a length of 100 mm and a thicknessof 2 mm was prepared. This metal base material was subjected toultrasonic washing in an alcohol solvent, and subsequently subjected toa sandblasting process so as to roughen the surface of the metal basematerial. The sandblasting process was carried out using Al₂O₃ abrasivegrains of #100 for 10 minutes.

Measurement using a surface-roughness measuring machine (HANDY SURFE-35B, manufactured by Tokyo Seimitsu Co., Ltd.) clarified that thesurface roughness Rz_(JIS) of the metal base material was 8.8 μm.

(3) Formation of Coat Film

The paint (0.7 g) prepared in the treatment of (1) was uniformly appliedto the surface of the metal base material obtained in the treatment of(2) by an electrocoating process. More specifically, the metal basematerial and an electrode plate were placed in the paint, and byallowing the metal base material and the electrode plate to functionrespectively as an anode and a cathode, and a voltage was applied.

The electrocoating process was carried out under conditions of a voltageof 100 V, a bath temperature of 26° C. to 32° C., and a time for 3minutes, while the paint was stirred with a rotary stirrer. The solidsconcentration of the paint was 15% by weight, and the pH thereof was ina range from 8.0 to 9.5.

(4) Drying and Curing

The metal base material coated with the paint in the treatment of (3)was heated at 160° C. for 60 minutes in a drying apparatus so that thecoat film of the paint formed on the surface of the metal base materialwas dried and cured.

(5) Degreasing

After the treatment of (4), the metal base material was heated in aheating furnace at 400° C. for 60 minutes so that the electrocoatingresin contained in the coat film was burned out.

The rate of the temperature rise from the heating temperature (160° C.)in the drying and curing treatment of (4) to the heating temperature(400° C.) in the present treatment was 4.0° C./minute.

(6) Firing

After the treatment of (5), the metal base material was heated in aheating furnace at 850° C. for 20 minutes so that the coat film wasfired.

The rate of the temperature rise from the heating temperature (400° C.)in the degreasing treatment of (5) to the heating temperature (850° C.)in the present treatment was 9.0° C./minute.

By carrying out the above-mentioned process, a baked sample of the paintin which a surface coating layer was formed on the metal base materialswas manufactured.

Example 2

A baked sample of the paint was manufactured in the same manner as inExample 1, except that in the treatment of (1) for manufacturing apaint, the amount of the powder of inorganic particles blended was 0part by weight so that the paint contained no inorganic particles, andthat the amount of the powder of inorganic glass particles blended was100 parts by weight.

Example 3

A baked sample of the paint was manufactured in the same manner as inExample 1, except that the rate of the temperature rise was accelerated.

The rate of the temperature rise from the heating temperature (160° C.)in the drying and curing treatment of (4) to the heating temperature(400° C.) in the degreasing treatment of (5) was 15.0° C./minute.

The rate of the temperature rise from the heating temperature (400° C.)in the degreasing treatment of (5) to the heating temperature (850° C.)in the firing treatment (6) was 25.0° C./minute.

Example 4

A baked sample of the paint was manufactured in the same manner as inExample 1, except that the rate of the temperature rise was accelerated.

The rate of the temperature rise from the heating temperature (160° C.)in the drying and curing treatment of (4) to the heating temperature(400° C.) in the degreasing treatment of (5) was 10.0° C./minute.

The rate of the temperature rise from the heating temperature (400° C.)in the degreasing treatment of (5) to the heating temperature (850° C.)in the firing treatment of (6) was 15.0° C./minute.

Example 5

A baked sample of the paint was manufactured in the same manner as inExample 1, except that the rate of the temperature rise was delayed.

The rate of the temperature rise from the heating temperature (160° C.)in the drying and curing treatment of (4) to the heating temperature(400° C.) in the degreasing treatment of (5) was 2.0° C./minute.

The rate of the temperature rise from the heating temperature (400° C.)in the degreasing treatment of (5) to the heating temperature (850° C.)in the firing treatment of (6) was 4.0° C./minute.

Example 6

A baked sample of the paint was manufactured in the same manner as inExample 1, except that in the treatment of (1) for manufacturing apaint, inorganic particles having an average particle size of 3.8 μm andinorganic glass particles having an average particle size of 4.3 μm wereused.

Reference Example 1

A baked sample of the paint was manufactured in the same manner as inExample 1, except the following. Namely, in the treatment of (1) formanufacturing a paint, methylcellulose (Methylcellulose 25 manufacturedby Kishida Chemical Co., Ltd.) was used instead of the anionicelectrocoating resin as the organic binder material. The two types ofthe inorganic particles respectively having an average particle size of3.8 μm and 4.3 μm were used. In the treatment of (3) for forming a coatfilm, the paint was applied by spray coating (mist coating) instead ofan electrocoating process. The firing treatment of (6) was carried outwithout carrying out the degreasing treatment of (5).

The rate of the temperature rise from the heating temperature (160° C.)in the drying and curing treatment of (4) to the heating temperature(850° C.) in the firing treatment of (6) was 9.0° C./minute.

Thereafter, by using a cutter, cuts were formed on the baked sample ofthe paint in the longitudinal and lateral directions in a grid patternat a density of 30 lines/cm so that concave portions were formed on thebaked sample of the paint.

Comparative Example 1

A baked sample of the paint was manufactured in the same manner as inReference Example 1; however, no concave portions were formed on thebaked samples of the paint. Here, the baked sample of ComparativeExample 1 corresponds to the conventional art (for example, thetechniques described in JP-A 2009-433213 and JP-A 2009-133214).

Comparative Example 2

A baked sample of the paint was manufactured in the same manner as inComparative Example 1, except that in the treatment of (1) formanufacturing a paint, the amount of the powder of inorganic particlesblended was 0 part by weight so that the paint contained no inorganicparticles, and that the amount of the powder of inorganic glassparticles blended was 100 parts by weight.

Tables 1A and 1B show the formulations of the paints and the conditionsfor manufacturing of the baked samples of the paints in Examples 1 to 6,Reference Example 1, and Comparative Examples 1 to 2.

TABLE 1A Paint Inorganic glass particles Average Inorganic particlesparticle Average Organic binder material size Compounding particle sizeTg Type (μm) Type ratio (μm) Type (° C.) Exampel 1 SiO₂—BaO—B₂O₃ type0.9 MnO₂—FeO—CuO—CoO 6:2:1:1 0.8 Electrocoating resin Anionic 25 Example2 SiO₂—BaO—B₂O₃ type 0.9 — — — Electrocoating resin Anionic 25 Example 3SiO₂—BaO—B₂O₃ type 0.9 MnO₂—FeO—CuO—CoO 6:2:1:1 0.8 Electrocoating resinAnionic 25 Example 4 SiO₂—BaO—B₂O₃ type 0.9 MnO₂—FeO—CuO—CoO 6:2:1:1 0.8Electrocoating resin Anionic 25 Example 5 SiO₂—BaO—B₂O₃ type 0.9MnO₂—FeO—CuO—CoO 6:2:1:1 0.8 Electrocoating resin Anionic 25 Example 6SiO₂—BaO—B₂O₃ type 4.3 MnO₂—FeO—CuO—CoO 6:2:1:1 3.8 Electrocoating resinAnionic 25 Reference SiO₂—BaO—B₂O₃ type 4.3 MnO₂—FeO—CuO—CoO 6:2:1:1 3.8Methylcellulose — — Example 1 Comparative SiO₂—BaO—B₂O₃ type 4.3MnO₂—FeO—CuO—CoO 6:2:1:1 3.8 Methylcellulose — — Example 1 ComparativeSiO₂—BaO—B₂O₃ type 4.3 — — — Methylcellulose — — Example 2

TABLE 1B Process Temperature Temperature rise rise Degreasing ~400° C.Firing ~850° C. (400° C.) [° C./min.] (850° C.) [° C./min.] Exampel 1Degreased 4.0 Fired 9.0 Example 2 Degreased 4.0 Fired 9.0 Example 3Degreased 15.0 Fired 25.0 Example 4 Degreased 10.0 Fired 15.0 Example 5Degreased 2.0 Fired 4.0 Example 6 Degreased 4.0 Fired 9.0 Reference Notdegreased — Fired 9.0 Example 1 Comparative Not degreased — Fired 9.0Example 1 Comparative Not degreased — Fired 9.0 Example 2

The following evaluations were carried out on the baked samples of thepaints of Examples 1 to 6, Reference Example 1, and Comparative Examples1 to 2. Tables 2A and 2B show the results.

TABLE 2A Thickness of Diameter of Density of the surface Shape ofconcave concave Film thickness of coating layer Concave concave portionportions concave portion (μm) portions portions (μm) (pcs/cm²) (μm)Example 1 6.2 Present Circle 102 about 10³ 3.1 (Not penetrated) Example2 5.6 Present Circle  93 about 10³ 2.8 (Not penetrated) Example 3 5.8Present Circle 960 about 10³ 0.0 (Penetrated) Example 4 6.3 PresentCircle 800 about 10³ 2.0 (Not penetrated) Example 5 6.5 Present Circle104 10 2.4 (Not penetrated) Example 6 5.8 Present Circle 112 about 10³2.8 (Not penetrated) Reference 6.1 Present Line 510 (about 10³) 3.5 (Notpenetrated) Example 1 Comparative 5.8 Not present — — — — Example 1Comparative 5.7 Not present — — — — Example 2

TABLE 2B Ervaluation results Thermal Emissivity impact Measuredresistance General value Evaluation Evaluation Evaluation Example 1 0.87Excellent Good Excellent Example 2 0.84 Good Good Good Example 3 0.82Passing Good Good Example 4 0.85 Good Good Good Example 5 0.82 PassingGood Good Example 6 0.82 Passing Good Good Reference 0.83 Good Low LowExample 1 Comparative 0.81 Low Low Poor Example 1 Comparative 0.80 PoorPoor Poor Example 2(Evaluation of Emissivity)

The emissivity of the baked samples of the paints of Examples 1 to 6,Reference Example 1, and Comparative Examples 1 to 2 was measured usingan emissivity meter D&S AERD manufactured by KEM.

Evaluation results of the emissivity in Tables 2A and 2B were givenbased on 5 ranks of “Excellent”, “Good”, “Passing”, “Low” and “Poor”. Inthis case, the rank “Low” indicates that there were no changes inemissivity in comparison with that of Comparative Example 1(conventional art). The rank “Passing” indicates that the emissivity wasslightly improved (0.01) in comparison with that of Comparative Example1 (conventional art). The rank “Good” indicates that the emissivity wasrelatively (0.02 to 0.05) improved in comparison with that ofComparative Example 1 (conventional art). The rank “Excellent” indicatesthat the emissivity was remarkably (0.06 or more) improved in comparisonwith that of Comparative Example 1 (conventional art). The rank “Poor”indicates that the emissivity became worse in comparison with that ofComparative Example 1 (conventional art).

The results of evaluations of emissivity show that Example 1 correspondsto “Excellent”, Examples 2 and 4 and Reference Example 1 correspond to“Good”, Examples 3, 5 and 6 correspond to “Passing”, Comparative Example1 corresponds to “Low”, and Comparative Example 2 corresponds to “Poor”.

(Evaluation of Heat Impact Resistance)

The baked samples of the paints of Examples 1 to 6, Reference Example 1,and Comparative Examples 1 to 2 each were heated in a heating furnace at850° C. for 10 minutes, and the resultant samples were put into water at25° C. without a cooling period of time, and visually observed as towhether or not any drop off or crack occurred in the surface coatinglayers (baked coatings formed of the paints).

Evaluation results of the heat impact resistance in Tables 1A and 1B aregiven based on 3 ranks of “Good”, “Low” and “Poor”. In this case, therank “Good” indicates that there were neither drops off nor cracks. Therank “Low” indicates that although there was no drop off, cracksoccurred. Moreover, the rank “Poor” indicates that cracks as well asdrop off occurred. Between drop off and cracks, since the drop offcauses more damages to the surface coating layer than the cracks, theoccurrence of drop off is determined as “Poor”.

The results of evaluations of heat impact resistance show that examples1 to 6 correspond to “Good”, Reference example 1 and Comparative Example1 correspond to “Low”, and Comparative Examples 2 corresponds to “Poor”.

(General Evaluation)

Based on the evaluations of emissivity and heat impact resistance, ageneral evaluation as shown in Tables 2A and 2B was given as to theresults of Examples 1 to 6, Reference Example 1, and ComparativeExamples 1 to 2.

The general evaluation is given based on 4 ranks of “Excellent”, “Good”,“Low” and “Poor”. In the case where the sample ranked “Excellent” in theevaluation of emissivity is ranked “Good” in the evaluation of heatimpact resistance, this case is evaluated as “Excellent”. In the casewhere the sample ranked “Good” or “Passing” in the evaluation ofemissivity is ranked “Good” in the evaluation of heat impact resistance,this case is evaluated as “Good”. In the case where the sample ranked“Good” in the evaluation of emissivity is ranked “Low” in the evaluationof heat impact resistance, this case is evaluated as “Low”. In the casewhere the sample ranked “Passing”, “Low” or “Poor” in the evaluation ofemissivity is ranked “Low” or “Poor” in the evaluation of heat impactresistance, this case is evaluated as “Poor”.

Here, the rank “Excellent” is the best evaluation and corresponds to theevaluation of Example 1. The rank “Good” is the second best evaluationand corresponds to the evaluations of Examples 2 to 6. The rank “Low” isthe third evaluation and corresponds to the evaluation of ReferenceExample 1. The rank “Poor” is the worst evaluation and corresponds tothe evaluations of Comparative Examples 1 and 2.

Along with the determination of the presence of the concave portions onthe surface coating layer and observation of the shape of the concaveportions, the film thickness (distance D in FIG. 3) of the surfacecoating layer, the diameter of the concave portion, the density of theconcave portions, and the film thickness of the concave portion (seedistance d in FIG. 3) were measured. The film thickness of the surfacecoating layer was obtained by measuring the cross section of each testpiece by SEM. The diameter of the concave portion was obtained bymeasuring the surface of each test piece by SEM. The density of theconcave portions was obtained by measuring the surface of each testpiece by SEM. The film thickness of the concave portion was obtained bymeasuring the cross section of each test piece by SEM. Tables 2A and 2Bshow the results.

The concave portion was determined to be “present” if a convex portionis present on the peripheral edge portion in the area (potential concaveportion) lower than the first reference surface.

The shape of the concave portion refers to a shape viewed in thedirection perpendicular to the surface of the base material. In the casewhere the shape is a virtually circular shape, the maximum length of astraight line drawn in the circle is defined as a diameter of theconcave portion, while in the case where the shape is a linear shape,the width of the straight line is defined as a diameter of the concaveportion.

In Tables 2A and 2B, “penetrated” refers to the fact that “the concaveportion penetrates the surface coating layer”, that is, the filmthickness of the concave portion d=0, and “Not penetrated” refers to thefact that “the concave portion does not penetrate the surface coatinglayer”, that is, the film thickness of the concave portion d>0.

In the general evaluation, Examples 1 to 6 and Reference Example 1 areranked higher than Comparative Examples 1 and 2.

This evaluation is presumably led by the fact that concave portions werepresent in Examples 1 to 6 and Reference Example 1, while no concaveportions were present in Comparative Examples 1 and 2.

In the general evaluation, Examples 1 to 6 are ranked higher thanReference Example 1.

This evaluation is presumably led by the fact that the concave portionsin Examples 1 to 6 each had a virtually circular shape, while theconcave portions in Reference Example 1 had a virtually linear shape.

The emissivity was 0.87 in Example 1 and 0.84 in Example 2, andtherefore, Example 1 is ranked higher than Example 2.

This result is presumably led by the fact that the paint containinginorganic particles was used in Example 1, while the paint containing noinorganic particles was used in Example 2.

Namely, the evaluation is based on the fact that, as described in theeffect (7) of the exhaust pipe of the embodiment of the presentinvention, in Example 1, use of the paint containing inorganic particlescaused infrared radiation from the inorganic particles in the surfacecoating layer. Moreover, presence of the inorganic particles reduced thefluidity of the inorganic glass particles softened during firing. Thissuppressed the flow of the coat film in the vicinity of the concaveportions. As a result, favorable concave portions were formed on thesurface of the surface coating layer.

In the evaluation of the emissivity, Example 4 (Emissivity: 0.85) wasranked higher than Example 3 (Emissivity: 0.82).

This evaluation is presumably based on the difference in the filmthickness (presence of penetration) of the concave portions. In thiscase, the film thickness of the concave portions was 0.0 μm (penetrated)in Example 3 and 2.0 μm (not penetrated) in Example 4.

Namely, the evaluation is based on the fact that, as described in theeffect (3) of the exhaust pipe of the embodiment of the presentinvention, in Example 4, the metal base material having low emissivitywas not exposed on the surface.

In the evaluation of the emissivity, Example 1 (Emissivity: 0.87) wasranked higher than Example 5 (Emissivity: 0.82).

This result is presumably based on the difference in the density of theconcave portions. In this case, the density was 10 pcs/cm² in Example 5and about 10³ pcs/cm² in Example 1.

The measured values of the emissivity was larger in Example 1(Emissivity: 0.87) than in Example 4 (Emissivity: 0.85).

This result is led by the difference in the film thickness of theconcave portions. In this case, the film thickness of the concaveportions was 2.0 μm (not penetrated) in Example 4 and 3.1 μm (notpenetrated) in Example 1.

Namely, this result is led by the fact that the distance between thesurface of the metal base material having low emissivity and the bottomof the concave portion was longer in Example 1 than in Example 4 (seeeffect (4) according to the present embodiment).

Moreover, in the evaluation of the emissivity, Example 1 (emissivity:0.87) was ranked higher than Example 6 (emissivity: 0.82).

This evaluation is led by the fact that, while Example 6 used inorganicglass particles having a large average particle size of 4.3 μm andinorganic particles having a large average particle size of 3.8 μm,Example 1 used inorganic glass particles having a small average particlesize of 0.9 μm and inorganic particles having a small average particlesize of 0.8 μm.

That is, this evaluation is led by the fact that, in Example 1, use ofthe inorganic glass particles having a small average particle size andinorganic particles having a small average particle size enabled tostabilize the inorganic glass particles and inorganic particles in thepaint solution, leading to formation of desirable concave portion on thesurface of the surface coating layer.

(Other Embodiments)

As described in the embodiments of the exhaust pipe of the embodiment ofthe present invention, the shape of the metal base material is desirablya substantially cylindrical shape. However, the shape of the metal basematerial is not limited to the substantially cylindrical shape, and maybe a substantially plate shape or a substantially semi-cylindricalshape, and the shape of the cross section may be a substantiallycircular shape, or may be other shapes, such as an substantiallyelliptical shape and a substantially polygonal shape.

The face of the metal base material to be coated with a paint is notnecessarily limited to the entire outer peripheral face of the metalbase material, and may be a part of the outer peripheral face of themetal base material.

However, in the case where only a part of the outer peripheral face ofthe metal base material is coated with the paint, the area of the partcoated with the paint is desirably about 10% or more, more desirablyabout 50% or more, and furthermore desirably about 80% or more of theentire area of the outer peripheral face of the metal base material. Inthe case where the area of the part coated with the paint is about 10%or more of the entire area of the outer peripheral face of the metalbase material, the area coated with the paint is not too small, theinternal temperature rise of the exhaust pipe is likely to beeffectively suppressed.

In the case of using a substantially cylindrical metal base material,the surface coated with the paint may be not the outer peripheral face,but the inner circumferential face of the metal base material. In thiscase, the outer peripheral face of the metal base material refers to asurface having a larger area of surfaces of the metal base material, andthe inner circumferential face of the metal base material refers to asurface having a smaller area thereof.

Moreover, both surfaces of the metal base material may be coated with apaint.

In the exhaust pipe of the embodiment of the present invention, presenceof a concave portion and a convex portion on the surface of the surfacecoating layer is an essential structural feature.

By combining the essential structural feature with various structuralelements described in the embodiments in detail (such as the kind ofinorganic glass base material, the kind of inorganic particles and thelike) appropriately, it becomes possible to obtain desired effects.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A heat releasing pipe comprising: a metal pipehaving an outer circumferential surface; a surface coating layerprovided on the outer circumferential surface of the metal pipe, thesurface coating layer containing an inorganic glass base material andhaving concave portions and convex portions on an outer surface of thesurface coating layer, the concave portions and the convex portionsbeing constructed using electrocoating with an electrocoating resin; theconcave portions having a virtually circular shape when seen in adirection perpendicular to the outer circumferential surface of themetal pipe and being lower than a first reference surface, the firstreference surface having an average height of the outer surface of thesurface coating layer; and the convex portions being located onperipheral edge portions of the concave portions and surrounding theconcave portions, the convex portions being higher than the firstreference surface.
 2. The heat releasing pipe according to claim 1,wherein the convex portions are higher than a second reference surface,the second reference surface having a height of (H_(max)−H×⅓), andwherein H_(max) represents a maximum height of the surface coatinglayer, H_(min) represents a minimum height of the surface coating layer,and H represents a difference between H_(max) and H_(min).
 3. The heatreleasing pipe according to claim 1, wherein d>0 is satisfied, andwherein H_(min) represents a minimum height of the surface coating layerand d represents a distance between a face having a height of H_(min)and the outer circumferential surface of the metal pipe.
 4. The heatreleasing pipe according to claim 3, wherein d≧about 2 μm is satisfied.5. The heat releasing pipe according to claim 1, wherein the concaveportions have a virtually circular shape having a diameter of about 3 μmto about 2000 μm when seen in a direction perpendicular to the outercircumferential surface of the metal pipe.
 6. The heat releasing pipeaccording to claim 1, wherein a density of the concave portions is about10 pcs/cm² to about 10⁷ pcs/cm².
 7. The heat releasing pipe according toclaim 1, wherein the surface coating layer further contains inorganicparticles.
 8. The heat releasing pipe according to claim 7, wherein theinorganic particles have an average particle size of not more than about3 μm.
 9. The heat releasing pipe according to claim 7, wherein theinorganic particles have an average interparticle distance of not morethan about 3 μm.
 10. The heat releasing pipe according to claim 7,wherein the inorganic particles are formed of an oxide of a transitionmetal.
 11. The heat releasing pipe according to claim 1, wherein theinorganic glass base material has a softening point of about 300° C. toabout 1000° C.
 12. The heat releasing pipe according to claim 1, whereinthe peripheral edge portions are an area of a graphic form F(2)excluding the concave portions in a view observed in a directionperpendicular to the outer circumferential surface of the metal pipe,and wherein F(1) represents a graphic form provided by the concaveportions, F(2) represents a graphic form similar to F(1) with a samecenter of gravity as F(1), and a ratio of similitude of F(1) to F(2) isF(1):F(2)=1:1.2.
 13. The heat releasing pipe according to claim 12,wherein about 60% or more of the peripheral edge portions of the concaveportions is occupied by the convex portions.